在使用TailwindCSS时，要给列表元素添加项目符号（即列表的点），我们可以使用一些基本的TailwindCSS实用类来实现。以下是一个简单的步骤，展示如何操作：

1. 首先，确保你的项目中已经安装并配置了TailwindCSS。
2. 接下来，使用`list-disc`类来为`<ul>`或`<ol>`元素添加项目符号。
3. 然后，使用`list-inside`类来设置列表的样式为内部，这样项目符号会显示在列表项内容的内侧，或者使用`list-outside`来使项目符号显示在外侧。
4. 最后，根据需要对列表项进行样式调整，比如使用`p-`, `m-`, `text-`等实用类来添加内边距、外边距或设置文本大小等。

下面是一个具体的例子：

```html
<ul class="list-disc list-inside m-4 p-2">
  <li class="mb-2 text-blue-600">第一项</li>
  <li class="mb-2 text-green-600">第二项</li>
  <li class="text-red-600">第三项</li>
</ul>
```

以上代码说明：
- `list-disc` 类会给每个 `<li>` 元素前面添加一个圆点作为项目符号。
- `list-inside` 类将项目符号放置在内容的内侧。
- `m-4` 类在整个 `<ul>` 元素上添加外边距。
- `p-2` 类在整个 `<ul>` 元素上添加内边距。
- `mb-2` 类在每个 `<li>` 元素的底部添加外边距，除了最后一个。
- `text-` 类用来设置不同的文字颜色。

通过这些步骤，你就可以仅使用TailwindCSS的实用类来获得列表元素的点，并且对其样式进行定制。

Just add `marker:text-color`, where `color` is the color you want:

    <ul class='marker:text-green list-outside list-disc ml-6'>
      <li>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean sollicitudin convallis viverra.</li>
      <li> Nunc nec gravida enim. Vestibulum venenatis luctus sem.</li>
      <li> Proin fringilla vel nulla eu molestie. Lorem ipsum dolor sit amet, consectetur adipiscing elit.</li>
    </ul>

You will have to specify it like this to achieve a colored bullet list.

    <li class="text-red-500">
      <div class="text-black">
        Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean sollicitudin convallis viverra.
      </div>
    </li>

I solved it like this:

    <ul class='list-outside list-disc ml-6'>
        <li class="text-red-500">
            <span class="text-black">Lorem ipsum dolor</span>
        </li>
        <li class="text-red-500">
            <span class="text-black">Nunc nec gravida enim.</span>
        </li>
    </ul>

Niche answer here, but for those wanting to achieve this in innerHTML in a React component, you can target the `li` element specifically with:

    .your-class {
      li::marker {
        @apply text-sky;
      }
    }
    

    <div className='your-class' dangerouslySetInnerHTML={{__html: yourContent}}/>
    

This answer is an approximation and depends on framework. If you're using Tailwind in React you may already be familiar with 'unresetting' your your base styles. This solution is for those who have done this.

More [here](https://www.swyx.io/tailwind-unreset/) and [here](https://tailwindcss.com/docs/adding-custom-styles#adding-base-styles).

This worked May 2023 (in the li class):

    <ul><li class="list-disc marker:text-red-800">text for this bullet point goes here</li></ul>
    

or, in the ul class (applies to all discs in the ul):

    <ul class="marker:text-indigo-900"><li class="list-disc ">text for this bullet point goes here</li></ul>

How to get colored bullet list dots just using tailwindcss utility classes

当您想要在 TailwindCSS 中实现宽度变化的动画效果时，您通常会通过组合几个不同的类来达到目的。具体来说，您会用到 TailwindCSS 的响应式设计特性、宽度工具类、过渡工具类和动画持续时间类。下面是实现宽度变化动画效果的步骤：

1. 定义初始和目标宽度
   首先，您需要定义元素的初始宽度和动画结束时的宽度。Tailwind 提供了一系列宽度类，比如 `w-0` (宽度为 0)，`w-full` (宽度为父元素的 100%)。

2. 使用过渡工具类
   为了使宽度变化更平滑，您可以使用 TailwindCSS 提供的 `transition` 类来定义元素的过渡属性。

3. 设置动画持续时间
   使用 `duration-` 前缀的类来设置动画的持续时间，例如 `duration-500` 会设置动画持续时间为 500 毫秒。

4. 触发动画
   您可以通过伪类（如 `hover:`）或 JavaScript 来触发宽度的变化。例如，您可以使用 `hover:` 类在鼠标悬停时改变元素的宽度。

下面是一个具体的例子，其中包含了一个会在鼠标悬停时展开宽度的动画：

```html

<div class="transition-width duration-500 hover:w-full w-0">
  
</div>
```

在这个例子中，`transition-width` 类定义了宽度变化的过渡效果，`duration-500` 类定义了动画的持续时间为 500 毫秒，`hover:w-full` 类表示当鼠标悬停在元素上时，其宽度会变成父元素的 100%，而 `w-0` 则定义了元素初始的宽度为 0。

请记住，TailwindCSS 默认情况下可能不包含所有宽度变化的过渡效果，您可能需要在您的 `tailwind.config.js` 文件中自定义 `theme.extend` 部分来添加您需要的过渡效果。

此外，如果您希望使用 JavaScript 来触发动画效果，可以通过添加或删除类来实现：

```javascript
const element = document.querySelector('.element-class');

element.addEventListener('mouseenter', () => {
  element.classList.toggle('w-full');
});

element.addEventListener('mouseleave', () => {
  element.classList.toggle('w-0');
});
```

上面的 JavaScript 代码片段展示了如何在鼠标移入和移出时切换宽度相关的类，从而触发宽度变化的动画效果。

<p>You need to perform a few steps to activate the behaviour you are looking for.</p>
<p>The first one is about extending you Tailwind theme via <code>tailwind.config.js</code>. You need to add the <code>transitionProperty</code> for <code>width</code>.</p>
<pre class="lang-js s-code-block"><code class="hljs language-javascript"><span class="hljs-variable language_">module</span>.<span class="hljs-property">exports</span> = {
    ...
    <span class="hljs-attr">theme</span>: {
        ...
        <span class="hljs-attr">extend</span>: {
            ...
            <span class="hljs-attr">transitionProperty</span>: {
                <span class="hljs-string">'width'</span>: <span class="hljs-string">'width'</span>
            },
        },
    },
}
</code></pre>
<p>The changes above create the <code>transition-width</code> class. Simply apply this one to your nav tag. In your specific case you can overwrite the <code>transition-all</code> class.</p>
<pre class="lang-html s-code-block"><code class="hljs language-xml"><span class="hljs-tag">&lt;<span class="hljs-name">nav</span> <span class="hljs-attr">class</span>=<span class="hljs-string">"text-white absolute md:relative flex-col min-h-full bg-black mt-24 md:mt-12 transition-width transition-slowest ease"</span> <span class="hljs-attr">:class</span>=<span class="hljs-string">"sidebarWidth"</span>&gt;</span>
</code></pre>
<p>The last step is quite easy: ensure that Tailwind is recreating the CSS. Afterwards you should see a smooth width transition in your project.</p>
<h2>Background to the Problem</h2>
<p>By default, the width and height transitions are not activated in Tailwind CSS. Here is the CSS that will be applied when using <code>transition</code> class.</p>
<pre class="lang-css s-code-block"><code class="hljs language-css"><span class="hljs-attribute">transition-property</span>: background-color, border-color, color, fill, stroke, opacity, box-shadow, transform;
<span class="hljs-attribute">transition-timing-function</span>: <span class="hljs-built_in">cubic-bezier</span>(<span class="hljs-number">0.4</span>, <span class="hljs-number">0</span>, <span class="hljs-number">0.2</span>, <span class="hljs-number">1</span>);
<span class="hljs-attribute">transition-duration</span>: <span class="hljs-number">150ms</span>;
</code></pre>
<p>Like you can see <code>width</code> and <code>height</code> are missing in <code>transition-property</code>.</p>
<p>You can find more information about it in the <a href="https://tailwindcss.com/docs/transition-property" rel="noreferrer">Tailwind documentation</a>.</p>

<p>You can make your own transition property like in tailwind.config.js :</p>
<p>Multiple properties :</p>
<pre><code>module.exports = {
  theme: {
    extend: {
      transitionProperty: {
        multiple: "width , height , backgroundColor , border-radius"
      }
   }
}
</code></pre>
<p>One property :</p>
<pre><code>module.exports = {
  theme: {
    extend: {
      transitionProperty: {
        width: "width"
      }
   }
}
</code></pre>

<p>If you're just looking to implement this in a single location &amp; it's not something you need throughout your entire theme (for that follow @insertcoin answer above), you can simply use an arbitrary one-off value as outlined here in the <a href="https://tailwindcss.com/docs/transition-property#arbitrary-values" rel="nofollow noreferrer">Tailwind Docs</a>.</p>
<p>The snippet below worked nicely for me:</p>
<pre><code>&lt;div class="transition-[width] duration-300"&gt;
  &lt;!-- ... --&gt;
&lt;/div&gt;
</code></pre>

How to do width transition in tailwind css

在Tailwind CSS中，您可以使用 `@apply` 指令在 CSS 文件中创建组件样式或使用变体，如 `hover`、`focus` 等来基于父类设置嵌套元素的样式。但是，标准的 Tailwind CSS 并没有提供直接基于父选择器设置嵌套元素样式的功能，这通常是因为 Tailwind 是一个实用工具优先的框架，它鼓励您直接在 HTML 元素上应用实用类。

然而，Tailwind CSS 提供了一个名为 `@layer` 的指令，您可以用它来组织您的 CSS 并利用 Tailwind 的 JIT 模式为嵌套选择器提供样式。这可以通过组合父选择器与子选择器实现。这里是一个示例：

```css
@layer components {
  .card {
    @apply bg-white rounded-lg shadow-md;

    .card-title {
      @apply text-lg font-bold;
    }

    .card-content {
      @apply text-base;
    }

    .card-footer {
      @apply text-sm text-gray-500;
    }
  }
}
```

在这个例子中，`.card-title`、`.card-content` 和 `.card-footer` 是嵌套在 `.card` 类下面的元素的样式。这些样式会在构建过程中与 Tailwind CSS 一起处理，并且他们将仅在父类 `.card` 存在时应用。

有一点需要注意的是，这种方法需要您在 `tailwind.config.js` 中启用 JIT 模式，因为标准模式下不支持这种复杂的嵌套。

另外，如果您愿意使用预处理器，如 PostCSS，您可以结合 Tailwind CSS 插件，如 `tailwindcss/nesting`，它可以让您使用标准的 CSS 嵌套语法来编写嵌套规则，这样可以使得基于父类设置嵌套元素的样式更为简洁明了。

This is possible in tailwind 3.1 with the addition of the arbitrary variants feature. The way you would do this is by using a selector on the parent div:

    <div className="card--primary bg-white [&>a]:text-black" >
      <a/>
      <a/>
    <div/>
    
    <div className="card--danger bg-orange [&>a]:text-white" >
      <a/>
      <a/>
    <div/>

**In case of Tailwind CSS**

Read the descriptive and a very brief documentation entry here: [Using arbitrary variants](https://tailwindcss.com/docs/hover-focus-and-other-states#using-arbitrary-variants).

As I can see you need to change the color of all the `<a>` links no matter how deeply they reside in the `<cards>`. Use an underscore between `&` and `a` selectors - `[&_a]:text-black`. This translates to:

    .card--primary {
          
      /* arbitrarily nested <a> links */
      a {
        color: black
      }
    }
    

On the other hand the Tailwind directive with `>` between `&` and `a` => `[&>a]:text-black` would result in this css (only the direct child `<a>` nodes would be styled):

    .card--primary {
          
      /* direct-child <a> links */
      > a {
        color: black
      }
    }
    

**Recap**: the resulting Tailwind HTML for your case:

    <div className="card--primary [&_a]:text-black" >
      <a></a> !-- will be black
      <div>
        <a></a> !-- will be black
      </div>
    <div/>
    
    <div className="card--danger [&_a]:text-white" >
      <a></a> !-- will be white
      <div>
        <a></a> !-- will be white
      </div>
    <div/>
    

That's it. I hope it is helpful.

In tailwind v3 you can have [arbitrary group selectors](https://tailwindcss.com/docs/hover-focus-and-other-states#arbitrary-groups). This allows you to specify a how a certain element behaves when nested beneath a parent class.

This is useful when you have a templated element needs to appear different under certain scenarios, but doesn't know anything about its parent context.

It requires the addition of the `.group` class, but then you can do something like this:

    <div class="card">
      <a href="#" class="text-black group-[.card--danger]:text-white">
        Normal Card
      </a>
    </div>
    <div class="card group card--danger bg-orange-600">
      <a href="#" class="text-black group-[.card--danger]:text-white">Danger Card</a>
    </div>
    

Here's an example on Tailwind's playground: [https://play.tailwindcss.com/IQ4W4BVdjw](https://play.tailwindcss.com/IQ4W4BVdjw)

How to style nested elements based on parent class using tailwind css

在Tailwind CSS中，要使元素的高度适应其内容，可以使用`h-auto`类，这相当于将CSS的`height`属性设置为`auto`，使得元素高度根据其内部内容自动调整。

例如，如果您有一个包含文本的`div`元素，想要它的高度根据内容来自动调整，那么可以这样写：

```html
<div class="h-auto">
  
  Lorem ipsum dolor sit amet, consectetur adipiscing elit...
</div>
```

而如果你是想要在Flexbox布局中，让项目的高度根据内容来调整，可以使用`items-start`或类似的对齐类来确保flex项根据内容定高度。请注意，在这种情况下，高度的自适应是Flexbox属性的一部分，而不是通过TailwindCSS的高度工具类来直接控制。

以下是一个使用Flexbox的例子，其中flex项的高度会根据内容大小而自适应：

```html
<div class="flex flex-col items-start">
  <div class="...">
    
    Lorem ipsum dolor sit amet...
  </div>
  <div class="...">
    
    Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua...
  </div>
</div>
```

在实际应用中，你可能需要结合其他类来满足布局的需求，例如用于设置宽度、内边距、外边距或其他样式。 Tailwind CSS的设计原则就是提供细粒度的工具类来帮助你快速构建自定义的UI组件。

you can create custom classes in your tailwind configuration for future use.

example:

    module.exports = {
      important: true,
      purge: ['./src/**/*.{js,jsx,ts,tsx}', './public/index.html'],
      darkMode: 'class', // or 'media' or 'class'
      theme: {
        extend: {
          colors: {
            ...
          },
          animation: {
           ...
          },
          width: {
            ...
          },
          margin: {
            ...
          },
          height: {
            76: '18rem',
            78: '19rem',
            82: '22rem',
            97: '28rem',
            98: '31rem',
            99: '38rem',
            100: '40rem',
            'fit-content': 'fit-content(20em)',
          },
          fontFamily: {
            mps: ['Clarkson', 'Helvetica', 'sans-serif']
          },
          flex: {
            2: '1 1 25%',
            3: '1 1 30%',
            4: '1 1 40%',
            5: '1 1 50%',
            6: '1 1 60%',
          }
        },
      },
      variants: {
        extend: {},
      },
      plugins: [],
    }

I managed to fix my issue by setting `h-full` in both `<textarea>` and setting `flex-none` in my problematic `<div>`, resulting in the following code:

    <form className="w-full flex h-96 gap-2 mt-8 flex-wrap">
         <textarea
              role="text-input"
              className="h-full resize-none flex-1"
              placeholder="INPUT"
         />
         <textarea
              role="text-output"
              className="h-full resize-none flex-1"
              placeholder="OUTPUT"
              readOnly
          />
          <div className="w-full flex flex-none"> // This is the troublesome div
              <button>
                    Submit!
              </button>
          </div>
    </form>

How to specify height fit content with tailwindcss

在Next.js中持久存储数据，通常会涉及以下几种策略：

### 1. 客户端存储
客户端存储通常用于存储用户偏好设置、会话状态等，并且通常只在客户端有效。

- **LocalStorage**: 可用于存储较小的数据片段，数据会在浏览器关闭后依然保留。
  - **例子**: 保存用户的主题偏好设置。
  
    ```javascript
    if (typeof window !== "undefined") {
      localStorage.setItem('theme', 'dark');
    }
    ```

- **SessionStorage**: 类似于LocalStorage，但它的存储生命周期是一次会话（session）。
  - **例子**: 存储用户在一个会话期间的数据，例如表单的部分输入。

    ```javascript
    if (typeof window !== "undefined") {
      sessionStorage.setItem('form_data', JSON.stringify({ name: 'John Doe' }));
    }
    ```

- **Cookies**: 与LocalStorage和SessionStorage不同，cookies可以配置过期时间，而且每次请求时都会发送到服务器。
  - **例子**: 存储用户登录信息，以便进行自动登录。

    ```javascript
    document.cookie = "token=123456; expires=Fri, 31 Dec 2021 23:59:59 GMT";
    ```

### 2. 服务器端存储
在服务器端，你可以使用各种数据库系统来持久化数据，这对于需要跨多个用户或会话存储数据的应用程序来说非常重要。

- **关系型数据库**: 如PostgreSQL, MySQL等，适合结构化数据存储。
  - **例子**: 存储用户帐户信息。

    ```javascript
    // 使用sequelize或其他ORM
    User.create({
      username: 'johndoe',
      password: 'securepassword'
    });
    ```

- **NoSQL数据库**: 如MongoDB, DynamoDB等，适合灵活的、半结构化数据。
  - **例子**: 存储用户生成的内容，如博客帖子。

    ```javascript
    // 使用mongoose或其他NoSQL数据库客户端
    const post = new Post({
      title: 'My First Blog Post',
      content: 'Content of the blog post...',
      author: 'johndoe'
    });

    post.save();
    ```

- **文件系统**: 适合存储大型数据，例如上传的文件。
  - **例子**: 存储用户上传的图片。

    ```javascript
    // Node.js中使用fs模块
    const fs = require('fs');

    fs.writeFile('/path/to/file.jpg', imageData, (err) => {
      if (err) throw err;
      console.log('The file has been saved!');
    });
    ```

### 3. 云服务
云服务，如AWS S3，Google Cloud Storage等，可以用于存储大量数据和静态资源。

- **例子**: 存储用户上传的视频文件。

  ```javascript
  // 使用AWS SDK
  const AWS = require('aws-sdk');
  const s3 = new AWS.S3();

  s3.upload({
    Bucket: 'my-bucket',
    Key: 'user-uploads/video.mp4',
    Body: videoStream
  }, (err, data) => {
    if (err) throw err;
    console.log(`File uploaded successfully at ${data.Location}`);
  });
  ```

### 4. API或微服务
如果你的应用程序是微服务架构的一部分，你可能会通过API调用远程服务来持久化数据。

- **例子**: 通过一个用户管理服务的API来创建新用户。

  ```javascript
  fetch('https://user-management-service.com/users', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      username: 'johndoe',
      email: 'john@example.com'
    }),
  }).then(response => response.json())
    .then(data => console.log(data));
  ```

在选择持久化数据的方法时，需要根据应用程序的需求、数据的类型和大小、安全性需求以及开发和运营的成本来决定。

### 5. IndexedDB
对于需要在客户端存储大量结构化数据的场景，IndexedDB是一个好的选择。它是一个低级的API，允许存储大量数据并且能够创建索引以高效地查询数据。

- **例子**: 存储大型数据集，例如一个离线可用的产品目录。

  ```javascript
  if (typeof window !== "undefined") {
    let request = window.indexedDB.open('ProductCatalog', 1);

    request.onerror = function(event) {
      console.error('Database error:', event.target.error);
    };

    request.onupgradeneeded = function(event) {
      let db = event.target.result;
      let objectStore = db.createObjectStore('products', { keyPath: 'id' });
      objectStore.createIndex('name', 'name', { unique: false });
      objectStore.createIndex('price', 'price', { unique: false });
      // 省略了数据填充部分
    };

    request.onsuccess = function(event) {
      let db = event.target.result;
      let transaction = db.transaction(['products'], 'readwrite');
      let objectStore = transaction.objectStore('products');
      objectStore.add({ id: 1, name: 'Laptop', price: 999 });
    };
  }
  ```

### 6. 环境变量和配置文件
对于一些不经常改变但需要持久化的配置数据，可以使用环境变量或配置文件。

- **例子**: 存储应用程序的配置设置，如API密钥。

  ```javascript
  // 在 .env 文件中
  DATABASE_URL=your-database-url

  // 在 Next.js 中读取环境变量
  const { DATABASE_URL } = process.env;
  ```

### 7. 第三方数据服务
你还可以使用第三方提供的数据服务，例如Firebase Realtime Database或Firestore，来处理数据存储和同步。

- **例子**: 使用Firebase Firestore来存储和同步应用数据。

  ```javascript
  import firebase from 'firebase/app';
  import 'firebase/firestore';

  if (!firebase.apps.length) {
    firebase.initializeApp({
      // 你的Firebase配置
    });
  }

  const db = firebase.firestore();

  const userRef = db.collection('users').doc('user-id');
  userRef.set({
    username: 'johndoe',
    email: 'john@example.com'
  });
  ```

在Next.js中，你还需要考虑数据存储的位置对于性能的影响。例如，如果你使用SSR（服务器端渲染），你可能需要确保数据检索过程是高效的，因为它会直接影响页面加载时间。

最后，无论选择哪种持久化方法，都要考虑到数据的安全性，确保敏感信息被正确加密，使用安全的传输方式，并且合理管理数据的访问权限。

Making a localStorage in Next.js persist is very frustrating, but it is possible, this is a link that I think would help one who wants to store some user preferences and settings, without setting or adding a modal and db to store the user's input. [How to persist data in Next.js with localStorage and no libraries.](https://esselr.vercel.app/thoughts/how-to-persist-and-access-data-in-next-js-using-localstorage-typescript)

Here is a [demo](https://nextlocalstorage.vercel.app/)

In a real app situation you would have a backend which would return your user data (automatically based on a token which is normally saved as a cookie). So each time your app is initialised you would make a request to get your user data and store it in your state management (redux, recoil, etc).

But in your case you want to make sure that you update your `localStorage` each time your user changes (I don't think there is any other case you want to update the localStorage). So your first useEffect should have `user` value in the dependency array like this:

      useEffect(() => {
        if (user) {
          localStorage.setItem('nickname', user);
        }
      }, [user]);
    

If the `user` value hasn't changed it doesn't make sense to update it with the same value.

Also I think you don't need second useEffect if you add the state setting to your first one (so in the end use just one useEffect which looks like this:

      useEffect(() => {
        if (user) {
          localStorage.setItem('nickname', user);
          setNick(user);
        }
      }, [user]);
      
    

)

在Next.js中持久存储数据，通常会涉及以下几种策略：

### 1. 客户端存储
客户端存储通常用于存储用户偏好设置、会话状态等，并且通常只在客户端有效。

- **LocalStorage**: 可用于存储较小的数据片段，数据会在浏览器关闭后依然保留。
  - **例子**: 保存用户的主题偏好设置。
  
    ```javascript
    if (typeof window !== "undefined") {
      localStorage.setItem('theme', 'dark');
    }
    ```

- **SessionStorage**: 类似于LocalStorage，但它的存储生命周期是一次会话（session）。
  - **例子**: 存储用户在一个会话期间的数据，例如表单的部分输入。

    ```javascript
    if (typeof window !== "undefined") {
      sessionStorage.setItem('form_data', JSON.stringify({ name: 'John Doe' }));
    }
    ```

- **Cookies**: 与LocalStorage和SessionStorage不同，cookies可以配置过期时间，而且每次请求时都会发送到服务器。
  - **例子**: 存储用户登录信息，以便进行自动登录。

    ```javascript
    document.cookie = "token=123456; expires=Fri, 31 Dec 2021 23:59:59 GMT";
    ```

### 2. 服务器端存储
在服务器端，你可以使用各种数据库系统来持久化数据，这对于需要跨多个用户或会话存储数据的应用程序来说非常重要。

- **关系型数据库**: 如PostgreSQL, MySQL等，适合结构化数据存储。
  - **例子**: 存储用户帐户信息。

    ```javascript
    // 使用sequelize或其他ORM
    User.create({
      username: 'johndoe',
      password: 'securepassword'
    });
    ```

- **NoSQL数据库**: 如MongoDB, DynamoDB等，适合灵活的、半结构化数据。
  - **例子**: 存储用户生成的内容，如博客帖子。

    ```javascript
    // 使用mongoose或其他NoSQL数据库客户端
    const post = new Post({
      title: 'My First Blog Post',
      content: 'Content of the blog post...',
      author: 'johndoe'
    });

    post.save();
    ```

- **文件系统**: 适合存储大型数据，例如上传的文件。
  - **例子**: 存储用户上传的图片。

    ```javascript
    // Node.js中使用fs模块
    const fs = require('fs');

    fs.writeFile('/path/to/file.jpg', imageData, (err) => {
      if (err) throw err;
      console.log('The file has been saved!');
    });
    ```

### 3. 云服务
云服务，如AWS S3，Google Cloud Storage等，可以用于存储大量数据和静态资源。

- **例子**: 存储用户上传的视频文件。

  ```javascript
  // 使用AWS SDK
  const AWS = require('aws-sdk');
  const s3 = new AWS.S3();

  s3.upload({
    Bucket: 'my-bucket',
    Key: 'user-uploads/video.mp4',
    Body: videoStream
  }, (err, data) => {
    if (err) throw err;
    console.log(`File uploaded successfully at ${data.Location}`);
  });
  ```

### 4. API或微服务
如果你的应用程序是微服务架构的一部分，你可能会通过API调用远程服务来持久化数据。

- **例子**: 通过一个用户管理服务的API来创建新用户。

  ```javascript
  fetch('https://user-management-service.com/users', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      username: 'johndoe',
      email: 'john@example.com'
    }),
  }).then(response => response.json())
    .then(data => console.log(data));
  ```

在选择持久化数据的方法时，需要根据应用程序的需求、数据的类型

在Next.js中持久存储数据，通常会涉及以下几种策略：

### 1. 客户端存储
客户端存储通常用于存储用户偏好设置、会话状态等，并且通常只在客户端有效。

- **LocalStorage**: 可用于存储较小的数据片段，数据会在浏览器关闭后依然保留。
  - **例子**: 保存用户的主题偏好设置。
  
    ```javascript
    if (typeof window !== "undefined") {
      localStorage.setItem('theme', 'dark');
    }
    ```

- **SessionStorage**: 类似于LocalStorage，但它的存储生命周期是一次会话（session）。
  - **例子**: 存储用户在一个会话期间的数据，例如表单的部分输入。

    ```javascript
    if (typeof window !== "undefined") {
      sessionStorage.setItem('form_data', JSON.stringify({ name: 'John Doe' }));
    }
    ```

- **Cookies**: 与LocalStorage和SessionStorage不同，cookies可以配置过期时间，而且每次请求时都会发送到服务器。
  - **例子**: 存储用户登录信息，以便进行自动登录。

    ```javascript
    document.cookie = "token=123456; expires=Fri, 31 Dec 2021 23:59:59 GMT";
    ```

### 2. 服务器端存储
在服务器端，你可以使用各种数据库系统来持久化数据，这对于需要跨多个用户或会话存储数据的应用程序来说非常重要。

- **关系型数据库**: 如PostgreSQL, MySQL等，适合结构化数据存储。
  - **例子**: 存储用户帐户信息。

    ```javascript
    // 使用sequelize或其他ORM
    User.create({
      username: 'johndoe',
      password: 'securepassword'
    });
    ```

- **NoSQL数据库**: 如MongoDB, DynamoDB等，适合灵活的、半结构化数据。
  - **例子**: 存储用户生成的内容，如博客帖子。

    ```javascript
    // 使用mongoose或其他NoSQL数据库客户端
    const post = new Post({
      title: 'My First Blog Post',
      content: 'Content of the blog post...',
      author: 'johndoe'
    });

    post.save();
    ```

- **文件系统**: 适合存储大型数据，例如上传的文件。
  - **例子**: 存储用户上传的图片。

    ```javascript
    // Node.js中使用fs模块
    const fs = require('fs');

    fs.writeFile('/path/to/file.jpg', imageData, (err) => {
      if (err) throw err;
      console.log('The file has been saved!');
    });
    ```

### 3. 云服务
云服务，如AWS S3，Google Cloud Storage等，可以用于存储大量数据和静态资源。

- **例子**: 存储用户上传的视频文件。

  ```javascript
  // 使用AWS SDK
  const AWS = require('aws-sdk');
  const s3 = new AWS.S3();

  s3.upload({
    Bucket: 'my-bucket',
    Key: 'user-uploads/video.mp4',
    Body: videoStream
  }, (err, data) => {
    if (err) throw err;
    console.log(`File uploaded successfully at ${data.Location}`);
  });
  ```

### 4. API或微服务
如果你的应用程序是微服务架构的一部分，你可能会通过API调用远程服务来持久化数据。

- **例子**: 通过一个用户管理服务的API来创建新用户。

  ```javascript
  fetch('https://user-management-service.com/users', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({
      username: 'johndoe',
      email: 'john@example.com'
    }),
  }).then(response => response.json())
    .then(data => console.log(data));
  ```

在选择持久化数据的方法时，需要根据应用程序的需求、数据的类型
和大小、安全性需求以及开发和运营的成本来决定。

### 5. IndexedDB
对于需要在客户端存储大量结构化数据的场景，IndexedDB是一个好的选择。它是一个低级的API，允许存储大量数据并且能够创建索引以高效地查询数据。

- **例子**: 存储大型数据集，例如一个离线可用的产品目录。

  ```javascript
  if (typeof window !== "undefined") {
    let request = window.indexedDB.open('ProductCatalog', 1);

    request.onerror = function(event) {
      console.error('Database error:', event.target.error);
    };

    request.onupgradeneeded = function(event) {
      let db = event.target.result;
      let objectStore = db.createObjectStore('products', { keyPath: 'id' });
      objectStore.createIndex('name', 'name', { unique: false });
      objectStore.createIndex('price', 'price', { unique: false });
      // 省略了数据填充部分
    };

    request.onsuccess = function(event) {
      let db = event.target.result;
      let transaction = db.transaction(['products'], 'readwrite');
      let objectStore = transaction.objectStore('products');
      objectStore.add({ id: 1, name: 'Laptop', price: 999 });
    };
  }
  ```

### 6. 环境变量和配置文件
对于一些不经常改变但需要持久化的配置数据，可以使用环境变量或配置文件。

- **例子**: 存储应用程序的配置设置，如API密钥。

  ```javascript
  // 在 .env 文件中
  DATABASE_URL=your-database-url

  // 在 Next.js 中读取环境变量
  const { DATABASE_URL } = process.env;
  ```

### 7. 第三方数据服务
你还可以使用第三方提供的数据服务，例如Firebase Realtime Database或Firestore，来处理数据存储和同步。

- **例子**: 使用Firebase Firestore来存储和同步应用数据。

  ```javascript
  import firebase from 'firebase/app';
  import 'firebase/firestore';

  if (!firebase.apps.length) {
    firebase.initializeApp({
      // 你的Firebase配置
    });
  }

  const db = firebase.firestore();

  const userRef = db.collection('users').doc('user-id');
  userRef.set({
    username: 'johndoe',
    email: 'john@example.com'
  });
  ```

在Next.js中，你还需要考虑数据存储的位置对于性能的影响。例如，如果你使用SSR（服务器端渲染），你可能需要确保数据检索过程是高效的，因为它会直接影响页面加载时间。

最后，无论选择哪种持久化方法，都要考虑到数据的安全性，确保敏感信息被正确加密，使用安全的传输方式，并且合理管理数据的访问权限。

How to persistently store data in next js

在Tailwind CSS中，设置整个页面的背景色可以通过在HTML的`<body>`标签中使用背景颜色工具类（utility classes）来完成。Tailwind 提供了一系列的背景色工具类，可以用来快速地应用颜色到元素上。以下是如何操作的步骤和一个例子：

1. **查找背景色工具类**：首先，需要选择一个背景色。Tailwind CSS 提供了丰富的颜色系统，有基础的颜色也有深色或浅色的变体。例如，假设你想要一个蓝色的背景，你可以选择 `bg-blue-500`。

2. **应用到 `<body>` 标签**：接下来，将这个工具类应用到 `<body>` 标签上。这样，整个页面的背景将被设置为选定的颜色。

3. **确保TailwindCSS已正确集成**：在实际编码之前，确保你的项目已经集成了TailwindCSS，并且你的配置文件（如`tailwind.config.js`）包含了你想使用的颜色。

### 示例

假设在你的TailwindCSS项目中，你想要将整个页面的背景设置为中等深度的蓝色（例如 `bg-blue-500`）。你可以这样编写你的HTML代码：

```html
<!doctype html>
<html lang="en">
<head>
  
  <link href="/path/to/your/tailwind.css" rel="stylesheet">
</head>
<body class="bg-blue-500">
  
</body>
</html>
```

在这个例子中，整个页面的背景将会是蓝色，因为我们给 `<body>` 标签添加了 `class="bg-blue-500"`。这样，无论页面内容有多长，背景色都会覆盖整个浏览器窗口。

请务必根据你的项目和设计要求选择合适的颜色类。如果你需要使用自定义颜色，你可以在 `tailwind.config.js` 文件中进行配置并创建一个自定义工具类。

### Short answer

change `h-screen` to `min-h-screen`.

* * *

### But why?

> `h-screen` :takes height of the viewport.

#### What happens when scrolled ?

     Now the `total height` is `viewport height + scrolled height`
                                    👆                  👆
                                   h-screen           extra height
    

So it doesn't expand itself to have height of the entire screen during scroll, So this can be overcomed by using `min-h-screen`, meaning minimum should be the viewport height and in the occurance of scroll , expand yourself to have maximum possible height.

How do you set a full page background color in tailwind css

Recoil 是一个用于在React应用中管理状态的库。在Recoil中，`atom`是存储状态的基本单元，通常用于存储应用的一部分状态。你可以将HTTP请求的结果设置为atom的默认值，但通常并非直接将请求结果作为默认值，而是使用`selector`来处理异步逻辑，并将该selector与atom结合使用。

在Recoil中处理HTTP请求，并将结果设置为atom的默认值的一般步骤如下：

1. 创建一个`selector`来执行HTTP请求。
2. 在`selector`的`get`属性中执行异步函数，如使用`fetch`。
3. 创建一个atom，其默认值设置为这个`selector`。

下面是一个如何实现这一流程的例子：

```javascript
import { atom, selector, useRecoilValue } from 'recoil';

// 定义selector，用于执行HTTP请求
const fetchDataSelector = selector({
  key: 'fetchData', // 唯一标识
  get: async ({ get }) => {
    try {
      const response = await fetch('https://api.example.com/data');
      const data = await response.json();
      return data; // 返回请求的结果
    } catch (error) {
      throw error;
    }
  }
});

// 定义atom，其默认值是上面定义的selector
const dataAtom = atom({
  key: 'data', // 唯一标识
  default: fetchDataSelector // 使用selector作为默认值
});

// 在组件中使用atom的值
const MyComponent = () => {
  // 使用useRecoilValue hook来读取atom的值
  const data = useRecoilValue(dataAtom);

  // 渲染数据或加载状态
  return (
    <div>
      {data ? <div>{JSON.stringify(data)}</div> : <div>Loading...</div>}
    </div>
  );
};
```

在上面的代码中，`fetchDataSelector`负责执行HTTP请求，`dataAtom`则使用这个selector作为其默认值。在组件`MyComponent`中，我们使用了`useRecoilValue` hook来读取`dataAtom`的值，这时会触发`fetchDataSelector`的执行，并在请求完成后提供数据。

需要注意的是，当组件首次渲染时，Recoil会执行selector中的异步操作。如果数据请求成功，Recoil会更新atom的值为请求结果，这样任何使用该atom的组件都将重新渲染并显示新的结果。如果请求失败，你应该处理错误，例如，你可以设置错误状态并在UI中展示给用户。

You can try creating a async selector and put into atom's default, like this:

    const mySelector = selector({
      key: 'mySelector',
      get: async ({get}) => {
        return await setService.getAll()
      }
    })
    
    const myAtom = atom({
      key:'myAtom',
      default: mySelector
    })

Here is a Typescript example, inspired by [this answer](https://stackoverflow.com/a/63904541/1928926).

    // Inside your React component
    
    const val = useRecoilValue(myAtom);
    const updateVal = useSetRecoilState(mySelector);
    
    // Update data with API call on form submit, and append new data to atom state
    const onFormSubmit: SubmitHandler<DataFormValues> = async (values) => {
        const createdData = await createDataWithApiCall();
        await updateData([createdData]);
    };
    

    // Inside recoil state file
    
    export const mySelector = selector({
        key: 'mySelector',
        get: async (): Promise<Array<Data>> => {
            // Set default value to API result
            const apiData = await callApi();
            return apiData;
        },
        set: ({ set, get }, newData) => {
            // Update state w/ new appended values
            const currentState = get(myAtom);
            const newState = [...currentState, ...newData as Data[]];
            set(myAtom, newState);
        },
    });
    
    export const myAtom = atom({
        key: "myAtom",
        default: mySelector,
    });
    

Context: Don't be required to refetch entire to-do list from API each time a user adds an item to a to-do list. Call the API to add the new item, and append the new item to recoil state. Useful when user wants to make edits as well.

How do i set the result of an api call as the default value for a recoil atom

在 Recoil 中，`atomFamily` 是一个工具函数，它允许我们创建一组相关的 atoms，每一个 atom 都有一个独特的参数作为标识。然而，Recoil 原生并没有直接提供一个函数可以一次性获取 `atomFamily` 中所有的元素。但是，我们可以通过跟踪使用过的参数来间接获取所有的元素。

要追踪一个 `atomFamily` 中所有的元素，你可以创建一个 Recoil selector，这个 selector 会追踪每个被创建和使用过的 atom。每次使用 `atomFamily` 的时候，你可以将其参数添加到一个全局的集合中，并通过这个集合来知道哪些 `atomFamily` 的成员是被访问过的。

例如，以下是如何实现这个功能的一个简单例子：

```javascript
import { atomFamily, selector, useRecoilValue } from 'recoil';

// 定义一个用于追踪访问过的 atomFamily 成员的集合
const atomFamilyParametersSet = new Set();

// 一个示例 atomFamily
const myAtomFamily = atomFamily({
  key: 'MyAtomFamily',
  default: param => {
    // 这里添加逻辑来确定你的默认值
    return defaultValueBasedOnParam(param);
  },
  effects_UNSTABLE: param => [
    ({setSelf, onSet}) => {
      // 当atomFamily的成员被创建时，将其参数添加到集合中
      atomFamilyParametersSet.add(param);
      onSet((newValue, oldValue, isReset) => {
        // 如果有必要，也可以在这里处理值的变化
      });
    },
  ],
});

// 定义一个 selector 来追踪和获取所有已访问的 atomFamily 成员
const allMyAtomFamilyMembers = selector({
  key: 'AllMyAtomFamilyMembers',
  get: ({ get }) => {
    // 将所有追踪过的参数转换成对应的 atomFamily 成员的值
    const allAtoms = Array.from(atomFamilyParametersSet).map(param =>
      get(myAtomFamily(param))
    );
    return allAtoms;
  },
});

// 使用 React 组件时，可以这样获取所有 atomFamily 成员的状态
const MyComponent = () => {
  const allMembers = useRecoilValue(allMyAtomFamilyMembers);
  // 渲染所有 atomFamily 成员的信息...
};
```

在这个例子中，我们创建了一个 `atomFamily`，并使用 `effects_UNSTABLE` 进行了一个效果的设置，在每次这个 atom 被创建时，我们会将其参数添加到 `atomFamilyParametersSet` 集合中。然后，我们定义了一个 `selector` 来获取和追踪所有已经访问过的 `atomFamily` 成员，我们利用这个 `selector` 来得到这些成员的状态。最后，在组件中使用 `useRecoilValue` 来获取所有成员的状态并进行渲染。请注意，这个方法只能跟踪到被实际使用过（即被 `get` 或 `set`）的 atomFamily 成员。未被使用的成员不会被添加到集合中。

Instead of using `useRecoilCallback` you can abstract it with `selectorFamily`.

    // atomFamily
    const mealsAtom = atomFamily({
      key: "meals",
      default: {}
    });
    
    const mealIds = atom({
      key: "mealsIds",
      default: []
    });
    
    // abstraction
    const meals = selectorFamily({
      key: "meals-access",
      get:  (id) => ({ get }) => {
          const atom = get(mealsAtom(id));
          return atom;
      },
      set: (id) => ({set}, meal) => {
          set(mealsAtom(id), meal);
          set(mealIds (id), prev => [...prev, meal.id)]);
      }
    });
    

Further more, in case you would like to support reset you can use the following code:

    // atomFamily
    const mealsAtom = atomFamily({
      key: "meals",
      default: {}
    });
    
    const mealIds = atom({
      key: "mealsIds",
      default: []
    });
    
    // abstraction
    const meals = selectorFamily({
      key: "meals-access",
      get:  (id) => ({ get }) => {
          const atom = get(mealsAtom(id));
          return atom;
      },
      set: (id) => ({set, reset}, meal) => {
          if(meal instanceof DefaultValue) {
            // DefaultValue means reset context
            reset(mealsAtom(id));
            reset(mealIds (id));
            return;
          }
          set(mealsAtom(id), meal);
          set(mealIds (id), prev => [...prev, meal.id)]);
      }
    });
    

If you're using Typescript you can make it more elegant by using the following [guard](https://www.typescriptlang.org/docs/handbook/advanced-types.html).

    import { DefaultValue } from 'recoil';
    
    export const guardRecoilDefaultValue = (
      candidate: unknown
    ): candidate is DefaultValue => {
      if (candidate instanceof DefaultValue) return true;
      return false;
    };
    

Using this guard with Typescript will look something like:

    // atomFamily
    const mealsAtom = atomFamily<IMeal, number>({
      key: "meals",
      default: {}
    });
    
    const mealIds = atom<number[]>({
      key: "mealsIds",
      default: []
    });
    
    // abstraction
    const meals = selectorFamily<IMeal, number>({
      key: "meals-access",
      get:  (id) => ({ get }) => {
          const atom = get(mealsAtom(id));
          return atom;
      },
      set: (id) => ({set, reset}, meal) => {
          if (guardRecoilDefaultValue(meal)) {
            // DefaultValue means reset context
            reset(mealsAtom(id));
            reset(mealIds (id));
            return;
          }
          // from this line you got IMeal (not IMeal | DefaultValue)
          set(mealsAtom(id), meal);
          set(mealIds (id), prev => [...prev, meal.id)]);
      }
    });

You have to track all ids of the `atomFamily` to get all members of the family. Keep in mind that this is not really a list, more like a map.

Something like this should get you going.

    // atomFamily
    const meals = atomFamily({
      key: "meals",
      default: {}
    });
    
    const mealIds = atom({
      key: "mealsIds",
      default: []
    });
    

When creating a new objects inside the family you also have to update the `mealIds` atom.

I usually use a `useRecoilCallback` hook to sync this together

      const createMeal = useRecoilCallback(({ set }) => (mealId, price) => {
        set(mealIds, currVal => [...currVal, mealId]);
        set(meals(mealId), {name: mealId, price});
      }, []);
    

This way you can create a meal by calling:

    createMeal("bananas", 5);
    

And get all ids via:

    const ids = useRecoilValue(mealIds);

You can use an `atom` to track the ids of each atom in the `atomFamily`. Then use a `selectorFamily` or a custom function to update the atom with the list of ids when a new atom is added or deleted from the `atomFamily`. Then, the atom with the list of ids can be used to extract each of the atoms by their id from the `selectorFamily`.

    // File for managing state
    
    
    //Atom Family
    export const mealsAtom = atomFamily({
      key: "meals",
      default: {},
    });
    //Atom ids list
    export const mealsIds = atom({
      key: "mealsIds",
      default: [],
    });
    

This is how the `selectorFamily` looks like:

    // File for managing state
    
    export const mealsSelector = selectorFamily({
      key: "mealsSelector",
      get: (mealId) => ({get}) => {
        return get(meals(mealId));
      },
      set: (mealId) => ({set, reset}, newMeal) => {
        // if 'newMeal' is an instance of Default value, 
        // the 'set' method will delete the atom from the atomFamily.
        if (newMeal instanceof DefaultValue) {
          // reset method deletes the atom from atomFamily. Then update ids list.
          reset(mealsAtom(mealId));
          set(mealsIds, (prevValue) => prevValue.filter((id) => id !== mealId));
        } else {
          // creates the atom and update the ids list
          set(mealsAtom(mealId), newMeal);
          set(mealsIds, (prev) => [...prev, mealId]);
        }
      },
    });
    

**Now, how do you use all this?**

*   Create a meal:

In this case i'm using current timestamp as the atom id with `Math.random()`

    // Component to consume state
    
    import {mealsSelector} from "your/path";
    import {useSetRecoilState} from "recoil";
    const setMeal = useSetRecoilState(mealsSelector(Math.random()));
    
    setMeal({
        name: "banana",
        price: 5,
    });
    
    

*   Delete a meal:

    // Component to consume state
    
    import {mealsSelector} from "your/path";
    import {DefaultValue, useSetRecoilState} from "recoil";
    
    const setMeal = useSetRecoilState(mealsSelector(mealId));
    setMeal(new DefaultValue());
    

*   Get all atoms from atomFamily:

Loop the list of ids and render Meals components that receive the id as props and use it to get the state for each atom.

    // Component to consume state, parent of Meals component
    
    import {mealsIds} from "your/path";
    import {useRecoilValue} from "recoil";
    
    const mealIdsList = useRecoilValue(mealsIds);
    
        //Inside the return function:
        return(
          {mealIdsList.slice()
              .map((mealId) => (
                <MealComponent
                  key={mealId}
                  id={mealId}
                />
              ))}
        );
    

    // Meal component to consume state
    
    import {mealsSelector} from "your/path";
    import {useRecoilValue} from "recoil";
    
    const meal = useRecoilValue(mealsSelector(props.id));
    

Then, you have a list of components for Meals, each with their own state from the `atomFamily`.

Here is how I have it working on my current project:

(For context this is a dynamic form created from an array of field option objects. The form values are submitted via a graphql mutation so we only want the minimal set of changes made. The form is therefore built up as the user edits fields)

    import { atom, atomFamily, DefaultValue, selectorFamily } from 'recoil';
    
    type PossibleFormValue = string | null | undefined;
    
    export const fieldStateAtom = atomFamily<PossibleFormValue, string>({
      key: 'fieldState',
      default: undefined,
    });
    
    export const fieldIdsAtom = atom<string[]>({
      key: 'fieldIds',
      default: [],
    });
    
    export const fieldStateSelector = selectorFamily<PossibleFormValue, string>({
      key: 'fieldStateSelector',
      get: (fieldId) => ({ get }) => get(fieldStateAtom(fieldId)),
      set: (fieldId) => ({ set, get }, fieldValue) => {
        set(fieldStateAtom(fieldId), fieldValue);
        const fieldIds = get(fieldIdsAtom);
        if (!fieldIds.includes(fieldId)) {
          set(fieldIdsAtom, (prev) => [...prev, fieldId]);
        }
      },
    });
    
    export const formStateSelector = selectorFamily<
      Record<string, PossibleFormValue>,
      string[]
    >({
      key: 'formStateSelector',
      get: (fieldIds) => ({ get }) => {
        return fieldIds.reduce<Record<string, PossibleFormValue>>(
          (result, fieldId) => {
            const fieldValue = get(fieldStateAtom(fieldId));
            return {
              ...result,
              [fieldId]: fieldValue,
            };
          },
          {},
        );
      },
      set: (fieldIds) => ({ get, set, reset }, newValue) => {
        if (newValue instanceof DefaultValue) {
          reset(fieldIdsAtom);
          const fieldIds = get(fieldIdsAtom);
          fieldIds.forEach((fieldId) => reset(fieldStateAtom(fieldId)));
        } else {
          set(fieldIdsAtom, Object.keys(newValue));
          fieldIds.forEach((fieldId) => {
            set(fieldStateAtom(fieldId), newValue[fieldId]);
          });
        }
      },
    });
    

The atoms are selectors are used in 3 places in the app:

In the field component:

    ...
    const localValue = useRecoilValue(fieldStateAtom(fieldId));
    const setFieldValue = useSetRecoilState(fieldStateSelector(fieldId));
    ...
    

In the save-handling component (although this could be simpler in a form with an explicit submit button):

    ...
    const fieldIds = useRecoilValue(fieldIdsAtom);
    const formState = useRecoilValue(formStateSelector(fieldIds));
    ...
    

And in another component that handles form actions, including form reset:

    ...
    const resetFormState = useResetRecoilState(formStateSelector([]));
    ...
    const handleDiscard = React.useCallback(() => {
      ...
      resetFormState();
      ...
    }, [..., resetFormState]);

How to get all elements from an atomfamily in recoil

在 React Recoil 中，通常我们会在组件内部使用 Recoil 的 `useRecoilState` 或 `useSetRecoilState` 钩子来更新 atom 原子状态。但是，在某些场景下，我们可能需要在组件外部，例如在一个异步函数或者一个普通的 JavaScript 模块中更新 Recoil 的状态。为了实现这一点，我们可以使用 Recoil 的 `RecoilRoot` 和 `atom` API 来创建全局状态，并使用 `useRecoilCallback` 钩子来创建一个可以在组件外部调用的更新函数。

下面是在组件外部更新 atom 状态的步骤：

1. **定义一个 atom：**

```jsx
import { atom } from 'recoil';

export const myAtom = atom({
  key: 'myAtom',
  default: 0, // 初始值
});
```

2. **在组件树中提供一个 RecoilRoot：**

```jsx
import React from 'react';
import { RecoilRoot } from 'recoil';
import App from './App';

function RootComponent() {
  return (
    <RecoilRoot>
      <App />
    </RecoilRoot>
  );
}
```

3. **使用 `useRecoilCallback` 创建可以从组件外部调用的回调函数：**

```jsx
import { useRecoilCallback } from 'recoil';
import { myAtom } from './store';

function useUpdateAtom() {
  const updateAtom = useRecoilCallback(({ set }) => (newValue) => {
    set(myAtom, newValue);
  }, []);

  return updateAtom;
}
```

4. **在组件内部调用 `useUpdateAtom` 并将返回的函数暴露给外部：**

```jsx
import React from 'react';
import { useUpdateAtom } from './useUpdateAtom';

function MyComponent() {
  const updateAtomFromOutside = useUpdateAtom();
  
  // 你现在可以将 updateAtomFromOutside 函数传递给外部或者注册为全局方法
  // 例如绑定到 window 对象，或者传递给需要调用更新操作的外部模块

  return (
    <div>
      <button onClick={() => updateAtomFromOutside(10)}>Update Atom</button>
    </div>
  );
}
```

5. **在组件外部使用该函数更新 atom 状态：**

```javascript
// 假设你已经在合适的地方获取到了 updateAtomFromOutside 函数
updateAtomFromOutside(20); // 这将会更新 myAtom 的值为 20
```

通过这种方式，我们可以轻松地在组件外部更新 Recoil 的状态，同时还能保持与 Recoil 状态管理库的整体架构兼容。这对于处理那些不直接绑定在 React 组件生命周期上的逻辑，如定时器、网络请求回调等情况特别有用。

As I pointed out in [another answer](https://stackoverflow.com/a/65198580/11783958), you generally don't want to run into this, but if you eventually really need to **update atoms outside of React** Components you might give a try to [**Recoil Nexus**](https://www.npmjs.com/package/recoil-nexus).

In the same file where you have your RecoilRoot you'll have something like:

    import React from 'react';
    import { RecoilRoot } from "recoil"
    import RecoilNexus from 'recoil-nexus'
    
    export default function App() {
      return (
        <RecoilRoot>
          <RecoilNexus/>
          
          {/* ... */}
          
        </RecoilRoot>
      );
    };
    
    
    export default App;
    

Then, wherever you need to read/update the values:

    import yourAtom from './yourAtom'
    import { getRecoil, setRecoil } from 'recoil-nexus'
    

Eventually you can get and set the values like this:

    
    const loading = getRecoil(loadingState)
    
    setRecoil(loadingState, !loading)
    
    

That's it!

_Disclaimer: I am the author of the library._

* * *

**Check this [CodeSandbox](https://githubbox.com/luisanton-io/simple-loader-with-nexus) for a live example.**

I think the only way (at least as of a few months ago) is a sort of hack where you include a non-rendering component that uses the recoil hooks and exports the provided functions from them.

See: [https://github.com/facebookexperimental/Recoil/issues/289#issuecomment-777249693](https://github.com/facebookexperimental/Recoil/issues/289#issuecomment-777249693)

Below is the file from my own project that achieves this, heavily based on that link above. All you need to do is put `<RecoilExternalStatePortal />` anywhere in your application tree that is guaranteed to _always_ render.

This seems like an omission in the Recoil API, IMHO.

    import React from 'react'
    import { Loadable, RecoilState, RecoilValue, useRecoilCallback, useRecoilTransactionObserver_UNSTABLE } from 'recoil'
    
    /**
     * Returns a Recoil state value, from anywhere in the app.
     *
     * Can be used outside of the React tree (outside a React component), such as in utility scripts, etc.
    
     * <RecoilExternalStatePortal> must have been previously loaded in the React tree, or it won't work.
     * Initialized as a dummy function "() => null", it's reference is updated to a proper Recoil state mutator when RecoilExternalStatePortal is loaded.
     *
     * @example const lastCreatedUser = getRecoilExternal(lastCreatedUserState);
     */
    export function getRecoilState<T>(recoilValue: RecoilValue<T>): T {
      return getRecoilLoadable(recoilValue).getValue()
    }
    
    /** The `getLoadable` function from recoil. This shouldn't be used directly. */
    let getRecoilLoadable: <T>(recoilValue: RecoilValue<T>) => Loadable<T> = () => null as any
    
    /**
     * Sets a Recoil state value, from anywhere in the app.
     *
     * Can be used outside of the React tree (outside a React component), such as in utility scripts, etc.
     *
     * <RecoilExternalStatePortal> must have been previously loaded in the React tree, or it won't work.
     * Initialized as a dummy function "() => null", it's reference is updated to a proper Recoil state mutator when RecoilExternalStatePortal is loaded.
     *
     * @example setRecoilExternalState(lastCreatedUserState, newUser)
     */
    export let setRecoilState: <T>(recoilState: RecoilState<T>, valOrUpdater: ((currVal: T) => T) | T) => void = () =>
      null as any
    
    /**
     * Utility component allowing to use the Recoil state outside of a React component.
     *
     * It must be loaded in the _app file, inside the <RecoilRoot> component.
     * Once it's been loaded in the React tree, it allows using setRecoilExternalState and getRecoilExternalLoadable from anywhere in the app.
     *
     * @see https://github.com/facebookexperimental/Recoil/issues/289#issuecomment-777300212
     * @see https://github.com/facebookexperimental/Recoil/issues/289#issuecomment-777305884
     * @see https://recoiljs.org/docs/api-reference/core/Loadable/
     */
    export function RecoilExternalStatePortal() {
      // We need to update the getRecoilExternalLoadable every time there's a new snapshot
      // Otherwise we will load old values from when the component was mounted
      useRecoilTransactionObserver_UNSTABLE(({ snapshot }) => {
        getRecoilLoadable = snapshot.getLoadable
      })
    
      // We only need to assign setRecoilExternalState once because it's not temporally dependent like "get" is
      useRecoilCallback(({ set }) => {
        setRecoilState = set
    
        return async () => {
          // no-op
        }
      })()
    
      return <></>
    }

I think the right way to handle this in react is with a custom hook/facade. That way you can keep code centralized, but share it with the components that need it, and in this case, include code that relies on hooks/being in a component. This article explains it fairly well:

[https://wanago.io/2019/12/09/javascript-design-patterns-facade-react-hooks/](https://wanago.io/2019/12/09/javascript-design-patterns-facade-react-hooks/)

But the basic idea is that you would create an custom `useAuth` hook, that would expose what you need:

    export function useAuth() {
      const [auth, setAuth] = useRecoilState(authAtom);
      const resetAuth = useResetRecoilState(authAtom);
      const authState = useMemo(() => {
        return {
          isAuthenticated: () => {
            return !!auth.accessToken && auth.expiresAt > new Date();
          },
        };
      }, [auth]);
      ...
      return { auth, authState, logout: resetAuth, /* and maybe more, error, login, etc */ };
    

and then use it in components that need auth:

    const AuthorizedComponent: FC<Props> = (props) => {
      const { auth, authState } = useAuth();

How to update atoms state in recoil js outside components react

在Mongoose中更新文档通常可以使用多种方法，这些方法包括 `Model.updateOne()`, `Model.updateMany()`, `Model.findOneAndUpdate()` 等。如果您想要在文档不存在的情况下插入新文档，可以使用 `Model.findOneAndUpdate()` 方法配合 `upsert` 选项。这里是一个例子：

首先，您需要一个模型来代表您的集合，例如：

```javascript
const mongoose = require('mongoose');
const { Schema } = mongoose;

const userSchema = new Schema({
  username: String,
  email: String,
  // 其他字段...
});

const User = mongoose.model('User', userSchema);
```

然后，如果您想更新一个用户的邮箱，可以这样做：

```javascript
// 假设我们有一个用户ID和一个新的邮箱地址
const userId = 'some-user-id';
const newEmail = 'newemail@example.com';

User.updateOne({ _id: userId }, { email: newEmail }, (err, result) => {
  if (err) {
    // 处理错误
  } else {
    // 更新成功，result.nModified 可以告诉你有多少文档被修改了
  }
});
```

如果您想要在更新时如果该文档不存在就创建一个新的文档，可以使用 `findOneAndUpdate()` 方法并设置 `upsert: true`，如下所示：

```javascript
// 假设我们有一个用户名和一个邮箱地址，并希望更新或创建用户
const username = 'johndoe';
const email = 'johndoe@example.com';

User.findOneAndUpdate(
  { username: username }, // 查询条件
  { username: username, email: email }, // 要更新或插入的数据
  { upsert: true, new: true }, // 选项: upsert 表示如果不存在则插入，new 表示返回更新后的文档
  (err, doc) => {
    if (err) {
      // 处理错误
    } else {
      // 如果文档存在，则doc是更新后的文档；如果文档不存在，则doc是新创建的文档
    }
  }
);
```

在这个例子中，`findOneAndUpdate` 方法会查找一个用户名为 'johndoe' 的文档，并更新其邮箱地址。如果没有找到匹配的文档，Mongoose将创建一个新的文档，其中包含提供的用户名和邮箱。

请注意，当使用 `upsert: true` 时，Mongoose 会默认按查询条件创建一个新的文档。如果你的查询条件包含字段之外的模型定义（比如上面的例子中的 `username`），你可能需要确保这些字段在模型定义中存在，否则它们可能不会被包含在新创建的文档中。此外，由于 `upsert` 操作是原子的，所以不需要担心在查询和插入之间有其他进程或线程插入相同的数据。

In Mongoose, you'd use `Person.update` [per the documentation](http://mongoosejs.com/docs/api.html#model_Model.update). In order to create a document if it doesn't already exist, you need to pass `{ upsert : true }` in the options hash as it defaults to `false`.

i.e.

    Person.update( { name : 'Ted' }, { name : 'Ted', age : 50 }, { upsert : true }, callback );

`collection.update` with `upsert:true`. See also [here](https://docs.mongodb.com/manual/reference/method/db.collection.update/).

`[db.collection.replaceOne(filter, replacement, options)]` with `upsert:true`

E.g. from [here](https://docs.mongodb.com/manual/reference/method/db.collection.replaceOne/):

    try {    db.restaurant.replaceOne(
                { "name" : "Pizza Rat's Pizzaria" },
                { "_id": 4, "name" : "Pizza Rat's Pizzaria", "Borough" : "Manhattan", "violations" : 8 },
                { upsert: true }    
             ); 
        }
    catch (e){ print(e); }

For python:

    import pymongo
    
    client = pymongo.MongoClient("mongodb_address")
    db = client.db_name
    collection = db[collection_name]
    
    # update 'data' if 'name' exists otherwise insert new document
    collection.find_one_and_update({"name": some_name},
                                   {"$set": {"data": some_data}},
                                   upsert=True)

How to update if exists otherwise insert new document

Mongoose 中的 `populate` 方法的作用是用于自动替换文档中的指定路径，将其从仅仅是一个外键（通常是 ObjectId）替换为实际的引用文档。这种操作在传统的 SQL 数据库中被称为“连接”（Join）操作。在 NoSQL 文档数据库中，如 MongoDB，这种操作并非由数据库引擎原生支持，而是通过 Mongoose 这样的 ODM（对象文档映射器）来模拟关系型数据库中的连接操作。

假设我们有两个 Mongoose 模型：一个是 `User`，一个是 `Post`。每个 `Post` 都是由某个 `User` 发布的。在 `Post` 模型中，我们可能会存储发布它的用户的 `ObjectId`。在不使用 `populate` 的情况下，当我们从数据库中查询 `Post` 文档时，我们只能看到用户的 `ObjectId`，我们不能直接获取到用户的详细信息。如果我们想要显示帖子旁边的用户信息，就需要进行两次查询：一次获取帖子，一次根据保存在帖子中的用户 `ObjectId` 获取用户信息。

使用 `populate`，我们可以在查询 `Post` 文档时告诉 Mongoose 自动地去获取并包含关联的 `User` 文档。举个例子：

```javascript
Post.find().populate('user').exec((err, posts) => {
  // 在这里，每个 `posts` 中的 'user' 字段现在不再是一个 ObjectId，
  // 而是一个完整的 `User` 文档。
});
```

这里的 `'user'` 是 `Post` 模型中定义的一个路径，它引用了 `User` 模型。`populate` 就是根据这个路径去找到对应的 `User` 文档，并将其包含在查询结果中。

使用 `populate` 可以大幅简化查询的逻辑，使得我们可以一次性获取到完整的数据，而不需要编写多个查询并手动组合它们的结果。然而，它也可能导致性能问题，因为每次 `populate` 都可能会导致额外的数据库查询，尤其是在有多个层级的引用时。因此，在使用 `populate` 时，需要注意性能和效率问题，有时可能需要考虑替代方案，比如使用 MongoDB 的 `$lookup` 聚合操作或者手动优化数据模型以减少对 `populate` 的依赖。

Populate will automatically replace the specified path in the document, with document(s) from other collection(s).

Your example

    Story.findOne({ title: Nintendo })
    

will return a Story of this kind:

    {
      _creator : A0jfdSMmEJj9, // example value
        title    : Nintendo,
        fans     : [r432i900fds09809n, fdsjifdsjfueu88] // example values
      }
    }
    

That kind of request would be enough when we don't care about the author or the fans, only their IDs.

But in the case where we need that `_creator`'s name, we would need to make another request to find it in the database.

In Mongoose we have function populate() that we can chain to our previous request to directly get that information in our answer without doing an additional request.

    Story.findOne({ title: Nintendo }).populate('_creator')
    

will return

    {
      _creator : {
           _id : A0jfdSMmEJj*9,
           name: Sai,
           age: 100,
           stories : [fdsfdsfdsew38u, 89hr3232, ...]
        },
        title    : Nintendo,
        fans     : [r432i900fds09809n, fdsjifdsjfueu88]
      }
    }
    

If that's too much information, and his age and name are enough but we don't want the stories that he wrote, populate can take another argument containing the field that we need.

    Story.findOne({ title: Nintendo }).populate('_creator', 'name age')
    

will return

    {
      _creator : {
           name: Sai,
           age: 100,
        },
        title    : Nintendo,
        fans     : [r432i900fds09809n, fdsjifdsjfueu88]
      }
    }

`populate()` function in mongoose is used for populating the data inside the reference. In your example `StorySchema` is having `_creator` field which will reference to the `_id` field which is basically the `ObjectId` of the mongodb document.

> populate() function can accept a string or an object as an input.

Where string is the field name which is required to be populated. In your case that is `_creator`. After mongoose found one doc from mongodb and the result of that is like below

    _creator: {
      name: "SomeName",
      age: SomeNumber,
      stories: [Set Of ObjectIDs of documents in stories collection in mongodb]
    },
    title: "SomeTitle",
    fans: [Set of ObjectIDs of documents in persons collection in mongodb]
    

> populate can also accept the object as an input.

You can find the documents of mongoose's `populate()` function here : [http://mongoosejs.com/docs/2.7.x/docs/populate.html](http://mongoosejs.com/docs/2.7.x/docs/populate.html) or [https://mongoosejs.com/docs/populate.html](https://mongoosejs.com/docs/populate.html)

.populate() is being used in order to bring only needed information.

**EXAMPLE without .populate()**

    User.findOne({ name: Bob })
    

Will return

    {
      Bob : {
        _id : dasd348ew,
        email: bob@example.com,
        age: 25,
        job: teacher,
        nationality: American,
      }
    }
    

**EXAMPLE with .populate()**

    User.findOne({ name: Bob }).populate("Bob", "job email")
    

Will return

    {
      Bob : {
        job: teacher,
        email: bob@example.com,
      }
    }

[This answer](https://stackoverflow.com/a/53002303/8784518) to this question is not complete if you need to populate across multiple levels. We can use it to get a list of a user's friends. But to retrieve friends of a user's friends we can do this:

    const userSchema = new Schema({
      name: String,
      friends: [{ type: ObjectId, ref: 'User' }]
    });
    
    User.
      findOne({ name: 'Val' }).
      populate({
        path: 'friends',
        // Get friends of friends - populate the 'friends' array for every friend
        populate: { path: 'friends' }
      });

What does populate in mongoose mean

在Node.js中使用Mongoose操作MongoDB时，我们经常需要处理大量的数据。为了有效地处理这些数据，我们可能会使用游标(cursor)。游标允许我们遍历查询结果，而不必一次性将所有结果加载到内存中。在Mongoose中，我们可以通过使用`find()`方法后接`cursor()`方法来获取一个游标。接下来，我们可以使用`foreach`循环来迭代游标中的每个文档。

下面是一个使用`foreach`游标的例子：

```javascript
const mongoose = require('mongoose');
const { Schema } = mongoose;

// 首先，定义模型的schema
const userSchema = new Schema({
  name: String,
  age: Number
});

// 然后，创建模型
const User = mongoose.model('User', userSchema);

// 连接到MongoDB数据库
mongoose.connect('mongodb://localhost:27017/myapp');

// 一旦连接到数据库，我们就可以通过游标来处理数据
mongoose.connection.on('open', function() {
  const cursor = User.find().cursor();

  // 使用foreach迭代游标
  cursor.eachAsync(function(user) {
    // 这里你可以对每个用户(user)进行操作
    console.log(user.name);
  })
  .then(() => {
    console.log('所有用户处理完毕');
    mongoose.disconnect(); // 处理完成后记得断开数据库连接
  })
  .catch(err => {
    console.error('处理过程中发生错误:', err);
  });
});
```

以上代码展示了如何在Mongoose中使用游标来迭代数据库中的用户集合。通过调用`eachAsync`方法，我们可以为每一个迭代到的文档执行一个异步操作，该操作在本例中是一个简单的控制台日志输出。处理完成后，我们通过`then`链来关闭数据库连接，这样做可以确保所有数据都已经被处理完毕。如果在迭代过程中遇到任何错误，`catch`块将捕获这些错误并输出。

这种方法特别适用于需要处理大量数据且不想一次性加载到内存中的情况，可以有效防止内存溢出问题。

The answer depends on the driver you're using. All MongoDB drivers I know have `cursor.forEach()` implemented one way or another.

Here are some examples:

[node-mongodb-native](https://github.com/mongodb/node-mongodb-native)
=====================================================================

    collection.find(query).forEach(function(doc) {
      // handle
    }, function(err) {
      // done or error
    });
    

[mongojs](https://github.com/mafintosh/mongojs)
===============================================

    db.collection.find(query).forEach(function(err, doc) {
      // handle
    });
    

[monk](https://github.com/LearnBoost/monk)
==========================================

    collection.find(query, { stream: true })
      .each(function(doc){
        // handle doc
      })
      .error(function(err){
        // handle error
      })
      .success(function(){
        // final callback
      });
    

[mongoose](http://mongoosejs.com/)
==================================

    collection.find(query).stream()
      .on('data', function(doc){
        // handle doc
      })
      .on('error', function(err){
        // handle error
      })
      .on('end', function(){
        // final callback
      });
    

Updating documents inside of `.forEach` callback
================================================

The only problem with updating documents inside of `.forEach` callback is that you have no idea when all documents are updated.

To solve this problem you should use some asynchronous control flow solution. Here are some options:

*   [async](https://github.com/caolan/async)
*   promises ([when.js](https://github.com/cujojs/when), [bluebird](https://github.com/petkaantonov/bluebird))

Here is an example of using `async`, using its [`queue` feature](https://github.com/caolan/async#queueworker-concurrency):

    var q = async.queue(function (doc, callback) {
      // code for your update
      collection.update({
        _id: doc._id
      }, {
        $set: {hi: 'there'}
      }, {
        w: 1
      }, callback);
    }, Infinity);
    
    var cursor = collection.find(query);
    cursor.each(function(err, doc) {
      if (err) throw err;
      if (doc) q.push(doc); // dispatching doc to async.queue
    });
    
    q.drain = function() {
      if (cursor.isClosed()) {
        console.log('all items have been processed');
        db.close();
      }
    }

Using the `mongodb` driver, and modern NodeJS with async/await, a good solution is to use [`next()`](http://mongodb.github.io/node-mongodb-native/3.1/api/Cursor.html#next):

    const collection = db.collection('things')
    const cursor = collection.find({
      bla: 42 // find all things where bla is 42
    });
    let document;
    while ((document = await cursor.next())) {
      await collection.findOneAndUpdate({
        _id: document._id
      }, {
        $set: {
          blu: 43
        }
      });
    }
    

This results in only one document at a time being required in memory, as opposed to e.g. the accepted answer, where many documents get sucked into memory, before processing of the documents starts. In cases of "huge collections" (as per the question) this may be important.

If documents are large, this can be improved further by using a [projection](http://mongodb.github.io/node-mongodb-native/3.1/api/Collection.html#find), so that only those fields of documents that are required are fetched from the database.

How can i use a cursor foreach in mongodb using node js

在Mongoose中，`findOne` 方法用于在MongoDB数据库中查询并返回匹配特定查询条件的单个文档。这个方法接受一个查询对象作为参数，该对象指定了我们希望查找的文档的条件。返回的是一个满足条件的文档，如果没有找到，则返回 `null`。

以下是一个使用 `findOne` 方法的例子：

假设我们有一个名为 `User` 的模型，它代表一个用户集合。我们想查询一个用户名为 "johndoe" 的用户：

```javascript
const mongoose = require('mongoose');
const User = mongoose.model('User', new mongoose.Schema({
  username: String,
  email: String,
  password: String
}));

async function findUserByUsername(username) {
  try {
    // 使用 findOne 方法查找一个用户名为 'johndoe' 的用户
    const user = await User.findOne({ username: username });

    if (user) {
      // 用户被找到
      console.log(`User found: ${user}`);
      return user;
    } else {
      // 用户未找到
      console.log('User not found.');
      return null;
    }
  } catch (error) {
    // 错误处理
    console.error('An error occurred:', error);
  }
}

// 调用函数并传入用户名 'johndoe'
findUserByUsername('johndoe');
```

在上面的代码示例中，我们首先引入了 `mongoose` 并定义了一个 `User` 模型，该模型具有用户名（username）、电子邮件（email）和密码（password）属性。

我们定义了一个异步函数 `findUserByUsername`，它接受一个用户名作为参数，然后使用 `findOne` 方法根据提供的用户名查询用户。查询参数是一个对象，即 `{ username: username }`，在这里我们希望 `username` 字段与传入的 `username` 参数相匹配。

该函数使用 `await` 关键字等待 `findOne` 的执行结果，这意味着在返回结果之前，JavaScript 的事件循环可以继续处理其他事情。如果查询成功，并且找到了对应的用户，它将打印出找到的用户并将其返回。如果没有找到用户，则输出 "User not found."。如果查询过程中发生错误，则会捕获该错误，并在控制台中打印出来。

Found the problem, need to use `function(err,obj)` instead:

    Auth.findOne({nick: 'noname'}, function(err,obj) { console.log(obj); });

Mongoose basically wraps mongodb's api to give you a pseudo relational db api so queries are not going to be exactly like mongodb queries. Mongoose **findOne** query returns a query object, **not a document**. You can either use a callback as the solution suggests or as of v4+ **findOne** returns a thenable so you can use .then or await/async to retrieve the document.

    // thenables
    Auth.findOne({nick: 'noname'}).then(err, result) {console.log(result)};
    Auth.findOne({nick: 'noname'}).then(function (doc) {console.log(doc)});
    
    // To use a full fledge promise you will need to use .exec()
    var auth = Auth.findOne({nick: 'noname'}).exec();
    auth.then(function (doc) {console.log(doc)});
    
    // async/await
    async function async auth() {
      const doc = await Auth.findOne({nick: 'noname'}).exec();
      return doc;
    }
    auth();
    
    

See the [docs](https://mongoosejs.com/docs/promises.html) if you would like to use a third party promise library.

In my case same error is there , I am using Asyanc / Await functions , for this needs to add AWAIT for findOne

    Ex:const foundUser = User.findOne ({ "email" : req.body.email });
    

above , foundUser always contains Object value in both cases either user found or not because it's returning values before finishing findOne .

    const foundUser = await User.findOne ({ "email" : req.body.email });
    

above , foundUser returns null if user is not there in collection with provided condition . If user found returns user document.

You might want to consider using `console.log` with the built-in "arguments" object:

    console.log(arguments); // would have shown you [0] null, [1] yourResult
    

This will always output all of your arguments, no matter how many arguments you have.

How to use the findone method in mongoose?

在Mongoose中，限制查询特定字段可以通过以下两种主要方法实现：投影（Projection）和`select`方法。

### 投影（Projection）

投影是在查询时指定哪些字段应该返回给用户。在MongoDB和Mongoose中，可以在查询时通过第二个参数定义投影。例如，假设你有一个用户模型`User`，你只想获取他们的`name`和`email`字段，你可以这样写：

```javascript
User.find({}, 'name email', function(err, users) {
  if (err) {
    // 处理错误
  } else {
    // 处理查询结果
  }
});
```

上面的代码中，`'name email'`就是一个投影，表示只返回`name`和`email`字段。如果你想排除某些字段，比如不想要`password`字段，可以在字段前加上`-`来表示排除：

```javascript
User.find({}, '-password', function(err, users) {
  if (err) {
    // 处理错误
  } else {
    // 处理查询结果
  }
});
```

### `select`方法

另一种方法是使用Mongoose查询的`select`方法。该方法允许你更链式地构建查询，并且可以更灵活地指定或排除字段。使用`select`方法时，你也可以使用空格分隔字段名称来指定需要返回的字段，或者使用`-`来排除字段。例如：

```javascript
User.find().select('name email').exec(function(err, users) {
  if (err) {
    // 处理错误
  } else {
    // 处理查询结果
  }
});
```

或者排除某个字段：

```javascript
User.find().select('-password').exec(function(err, users) {
    if (err) {
        // 处理错误
    } else {
        // 处理查询结果
    }
});
```

在这个例子中，我们使用了链式写法，首先是`User.find()`来初始化查询，接着是`.select('name email')`来指定返回的字段，最后是`.exec()`来执行查询并处理结果。

值得注意的是，使用排除法时，默认情况下`_id`字段会被包含，除非你显式地排除它。如果你不想返回`_id`字段，你可以这样写：

```javascript
User.find().select('name email -_id').exec(function(err, users) {
    if (err) {
        // 处理错误
    } else {
        // 处理查询结果
    }
});
```

这些方法也可以与其他查询条件和选项结合使用，以进行更复杂的查询。通过这样的方式，你可以精确控制在Mongoose查询中返回哪些数据，以及如何返回这些数据。

the [docs](http://mongoosejs.com/docs/api.html#model_Model.find) say you can achieve this like so:

**Mongoose v4.0**

    // Retrieving only certain fields
    
    Model.find({}, 'first last', function (err, docs) {
    
    });
    

**old outdated API**

    // Retrieving only certain fields
    
    Model.find({}, ['first', 'last'], function (err, docs) {
      // docs is an array of partially-`init`d documents
      // defaults are still applied and will be "populated"
    });
    

so you can do this without `select()`.

this is another way: [queries in mongoose](http://mongoosejs.com/docs/queries.html)

    Transaction.find({username : user.username})
    .select('uniqueId confirmation_link item_name timeout username')
    .exec(function(err, txs) {
            console.log(txs);
    });

Now there is a shorter way of doing this (not using `.select` and not using an array), just passing the fields separate by spaces as the second argument

    User.find({}, 'first last', function (err, usr) {
        //Got the result, saved a few bytes of code
    });
    

[The Docs](http://mongoosejs.com/docs/api.html#model_Model.find)

Select method is used to select which fields are to be returned in the query result, excluding select means we want all the other fields to be returned, here is simple usage as per the docs.

    // include a and b, exclude other fields  
    query.select('a b');
    
    // exclude c and d, include other fields  
    query.select('-c -d');
    

More information here, [https://mongoosejs.com/docs/api.html#query\_Query-select](https://mongoosejs.com/docs/api.html#query_Query-select)

To retrieve certain fields without retrieving the '\_id' you can specify to exclude it

    Model.find({}, {'Username':1, '_id':0}, function ( err, docs ){}.....

How to using the select method in mongoose?

在Mongoose中，如果你想同时更新多个文档，可以使用`updateMany`方法。这个方法允许你根据指定的查询条件来更新所有匹配的文档。下面是一个例子：

假设有一个名为`User`的模型，该模型代表一个用户集合，我们想要将所有用户的`isActive`字段从`false`更新为`true`。

```javascript
const mongoose = require('mongoose');
const User = mongoose.model('User', new mongoose.Schema({
  isActive: Boolean
}));

async function updateMultipleUsers() {
  try {
    const filter = { isActive: false }; // 定义匹配条件
    const update = { isActive: true };  // 定义更新的内容

    // 使用updateMany将所有匹配filter的文档的isActive字段更新为true
    const result = await User.updateMany(filter, update);

    // result会包含更新操作的结果，例如匹配的文档数和实际更新的文档数
    console.log(result);
  } catch (error) {
    console.error('Error updating users:', error);
  }
}

// 调用函数以执行更新
updateMultipleUsers();
```

在这个例子中，我们首先定义了一个用于匹配要更新的文档的`filter`对象，这里它只有一个属性`isActive`，我们要匹配所有`isActive`为`false`的文档。然后，我们定义了一个`update`对象来指定要对匹配的文档进行的更新操作，即将`isActive`设置为`true`。

接着，我们调用`User.updateMany`方法，并传入这两个对象。这个方法返回一个Promise，所以我们可以使用`await`关键字等待它的完成。完成后，它会返回一个包含操作结果的对象，其中包括了匹配的文档数量和实际更新的文档数量。

如果需要应用更复杂的更新操作，比如使用MongoDB的更新操作符（例如`$set`，`$inc`等），你可以在`update`对象中包含这些操作符。此外，还可以传入一个选项对象来自定义操作的行为，例如是否要返回更新后的文档等。

Currently I believe that `update()` in Mongoose has some problems, see: [https://groups.google.com/forum/#%21topic/mongoose-orm/G8i9S7E8Erg](https://groups.google.com/forum/#%21topic/mongoose-orm/G8i9S7E8Erg) and [https://groups.google.com/d/topic/mongoose-orm/K5pSHT4hJ\_A/discussion](https://groups.google.com/d/topic/mongoose-orm/K5pSHT4hJ_A/discussion).

However, check the docs for update: [http://mongoosejs.com/docs/api.html](http://mongoosejs.com/docs/api.html) (its under Model). The definition is:

**Earlier Solution(Depreciated after mongoose 5+ version)**

    Model.update = function (query, doc, options, callback) { ... }
    

You need to pass the options inside an object, so your code would be:

    Model.update = function ({}, {cid: ''}, {multi: true}, function(err) { ... });
    

**New Solution**

    Model.updateMany = function (query, doc, callback) { ... }
    
    Model.updateMany = function ({}, {cid: ''}, function(err) { ... });
    

I believe that Mongoose wraps your cid in a $set, so this is not the same as running that same update in the mongo shell. If you ran that in the shell then all documents would be replaced by one with a single `cid: ''`.

Those answers are deprecated. This is the actual solution:

    Device.updateMany({}, { cid: '' });

You have to use the multi: true option

    Device.update({},{cid: ''},{multi: true});

as mentioned in the [mongoose documents](https://docs.mongodb.com/manual/reference/method/db.collection.updateMany/) this is how we do this:

`db.collection.updateMany(condition, update, options, callback function)`

so this is an example based on the docs:

        // creating arguments
        let conditions = {};
        let update = {
            $set : {
          title : req.body.title,
          description : req.body.description,
          markdown : req.body.markdown
          }
        };
        let options = { multi: true, upsert: true };
    
        // update_many :)
        YourCollection.updateMany(
    
          conditions, update, options,(err, doc) => {
            console.log(req.body);
            if(!err) {
              res.redirect('/articles');
            }
            else {
              if(err.name == "ValidationError"){
                handleValidationError(err , req.body);
                res.redirect('/new-post');
              }else {
                res.redirect('/');
              }
            }
          });
    

this worked fine for me, I hope it helps :)

await Device.updateMany({_id: {$in: cid}},{ $set: {columnNameHere: "columnValueHere"}},{multi:true,upsert: true,new: true});

How can i update multiple documents in mongoose

在使用Mongoose进行数据检索时，您可以通过在查询中添加特定的选项来排除某些字段。这是通过在查询的`select`方法中设置字段为`0`或者使用`-`符号排除字段来实现的。以下是几个示例：

### 示例 1：使用对象字面量排除字段

```javascript
// 假设我们有一个User模型
User.find({ /* 查询条件 */ })
    .select({ password: 0, __v: 0 }) // 排除password和__v字段
    .exec((err, users) => {
      // 处理结果
    });
```

在这个例子中，我们在`select`方法中使用一个对象，其中将不想包含的字段设置为`0`。

### 示例 2：使用字符串排除字段

```javascript
User.find({ /* 查询条件 */ })
    .select('-password -__v') // 使用字符串，以-开头表示排除字段
    .exec((err, users) => {
      // 处理结果
    });
```

在这个例子中，我们在`select`方法中使用了一个字符串，字段名前加上`-`符号表示排除该字段。

### 示例 3：在查询构造器中直接指定

```javascript
User.find({ /* 查询条件 */ }, '-password -__v', (err, users) => {
    // 处理结果
});
```

在这个例子中，我们直接在`find`方法的第二个参数中指定排除的字段。

### 示例 4：使用`projection`对象在`findOne`中排除字段

```javascript
User.findOne({ /* 查询条件 */ }, { password: 0, __v: 0 }, (err, user) => {
    // 处理结果
});
```

在这个例子中，我们在`findOne`方法中使用了`projection`对象来排除某些字段。

### 示例 5：链式查询中排除字段

```javascript
User.find({ /* 查询条件 */ })
    .select('-password')
    .lean()
    .exec((err, users) => {
      // 处理结果
    });
```

在这个例子中，我们展示了如何在链式查询中使用`select`方法排除字段。

这种排除字段的方法是出于性能和安全性的考虑，比如你不希望将密码、用户IDs以及其他敏感信息发送给客户端。

Another way to handle this on the schema level is to override toJSON for the model.

    UserSchema.methods.toJSON = function() {
      var obj = this.toObject()
      delete obj.passwordHash
      return obj
    }
    

I came across this question looking for a way to exclude password hash from the json i served to the client, and `select: false` broke my verifyPassword function because it didn't retrieve the value from the database at all.

The documented way is

    UserSchema.set('toJSON', {
        transform: function(doc, ret, options) {
            delete ret.password;
            return ret;
        }
    });
    

**UPDATE - You might want to use a white list:**

    UserSchema.set('toJSON', {
        transform: function(doc, ret, options) {
            var retJson = {
                email: ret.email,
                registered: ret.registered,
                modified: ret.modified
            };
            return retJson;
        }
    });

Come across your question when I was trying to find a similar answer with pymongo. It turns out that in mongo shell, with the find() function call, you can pass a second parameter which specifies how the result document looks like. When you pass a dictionary with attribute's value being 0, you are excluding this field in all the document that come out of this query.

In your case, for example, the query will be like:

    db.user.find({an_attr: a_value}, {_creator: 0});
    

It will exclude \_creator parameter for you.

In pymongo, the find() function is pretty much the same. Not sure how it translate to mongoose though. I think it's a better solution compare to manually delete the fields afterwards.

Hope it helps.

I would use the lodash utilities [_.pick()_](http://lodash.com/docs#pick) _or [](http://lodash.com/docs#omit)_ [.omit()](http://lodash.com/docs#omit)

    var _ = require('lodash');
    
    app.post('/example.json', function (req, res) {
        var user = new User({name: 'John', age: 45, _creator: 'some ObjectId'});
        user.save(function (err) {
            if (err) throw err;
            // Only get name and age properties
            var userFiltered = _.pick(user.toObject(), ['name', 'age']);
            res.json(200, {user: user});
        });
    });
    

The other example would be:

    var _ = require('lodash');
    
    app.post('/example.json', function (req, res) {
        var user = new User({name: 'John', age: 45, _creator: 'some ObjectId'});
        user.save(function (err) {
            if (err) throw err;
            // Remove _creator property
            var userFiltered = _.omit(user.toObject(), ['_creator']);
            res.json(200, {user: user});
        });
    });

You can call [`toObject()`](http://mongoosejs.com/docs/api.html#document_Document-toObject) on the document to convert it to a plain JS object that you can freely modify:

    user = user.toObject();
    delete user._creator;
    res.json(200, {user: user});

How to exclude some fields from the document in mongoose?

Mongoose 是一个面向 MongoDB 的对象数据模型（ODM）库，它为在 Node.js 中使用 MongoDB 提供了便捷的 API。在Mongoose中，`save`、`insert`和`create`函数都用于将数据保存到MongoDB数据库中，但它们各自的使用场景和工作方式略有不同。

### `save` 方法
`save` 方法是Mongoose模型实例上的一个方法。它用于将一个模型实例（document）保存到数据库中。如果该模型实例是新创建的，则执行插入（insert）操作；如果该模型实例已经存在于数据库中（通常是通过查询得到的），则执行更新（update）操作。

**示例：**
```javascript
const kitty = new Cat({ name: 'Zildjian' });
kitty.save((err, savedKitty) => {
  if (err) return console.error(err);
  console.log(savedKitty.name + ' saved to database.');
});
```

### `insert` 方法
`insert` 是MongoDB原生驱动的方法，Mongoose 通过 `Model.collection.insert` 或者 `Model.insertMany` 方法暴露了这一功能。这个方法通常用于批量插入多个文档到数据库中，不会进行模型的验证（validation），不会应用默认值，并且不会执行Mongoose的中间件（middleware）。

**示例：**
```javascript
Cat.insertMany([{ name: 'Kitty' }, { name: 'Catnip' }], (err, docs) => {
  if (err) return console.error(err);
  console.log('Multiple cats inserted to database.');
});
```

### `create` 方法
`create` 方法是一个模型（model）上的静态方法，它不仅可以创建单个文档，也可以创建多个文档，并将它们保存到数据库中。与 `insertMany` 不同，`create` 方法会进行模型验证，应用模型的默认值，并且可以触发Mongoose的中间件。

**示例：**
```javascript
Cat.create({ name: 'Fluffy' }, (err, createdCat) => {
  if (err) return console.error(err);
  console.log(createdCat.name + ' created and saved to database.');
});
```

或者创建多个文档：
```javascript
Cat.create([{ name: 'Fluffy' }, { name: 'Paws' }], (err, createdCats) => {
  if (err) return console.error(err);
  createdCats.forEach(cat => console.log(cat.name + ' created and saved to database.'));
});
```

### 总结
- **save**: 用于保存单个文档，可以是新文档（insert）也可以是更新已有文档（update），执行模型验证、应用默认值，并触发中间件。
- **insert**: 通过MongoDB驱动提供的能力，用于批量插入文档，不进行Mongoose层面的验证、不应用默认值，不触发中间件。
- **create**: 创建一个或多个文档并保存，执行模型验证、应用默认值，并触发中间件，适合需要验证和应用模型默认值的场景。

在实际应用中，选择哪一个方法取决于具体的场景和需求。例如，如果需要批量插入数据且不关心验证和默认值，可能会选择 `insertMany`。如果在插入数据的同时需要验证和应用默认值，则可能会选择 `create`。而 `save` 通常用于处理单个文档，并且在已有实例的基础上进行更新操作。

The `.save()` is an instance method of the model, while the `.create()` is called directly from the `Model` as a method call, being static in nature, and takes the object as a first parameter.

    var mongoose = require('mongoose');
    
    var notificationSchema = mongoose.Schema({
        "datetime" : {
            type: Date,
            default: Date.now
        },
        "ownerId":{
            type:String
        },
        "customerId" : {
            type:String
        },
        "title" : {
            type:String
        },
        "message" : {
            type:String
        }
    });
    
    var Notification = mongoose.model('Notification', notificationsSchema);
    
    
    function saveNotification1(data) {
        var notification = new Notification(data);
        notification.save(function (err) {
            if (err) return handleError(err);
            // saved!
        })
    }
    
    function saveNotification2(data) {
        Notification.create(data, function (err, small) {
        if (err) return handleError(err);
        // saved!
        })
    }
    

Export whatever functions you would want outside.

More at the [Mongoose Docs](http://mongoosejs.com/docs/models.html), or consider reading the reference of the [`Model`](http://mongoosejs.com/docs/api.html#Model) prototype in Mongoose.

**TLDR: Use Create (save is expert-mode)**

The main difference between using the create and save methods in Mongoose is that create is a convenience method that automatically calls new Model() and save() for you, while save is a method that is called on a Mongoose document instance.

When you call the create method on a Mongoose model, it creates a new instance of the model, sets the properties, and then saves the document to the database. This method is useful when you want to create a new document and insert it into the database in one step. This makes the creation an atomic transaction. Therefore, the save method leaves the potential to create inefficiencies/inconsistencies in your code.

On the other hand, the save method is called on an instance of a Mongoose document, after you have made changes to it. This method will validate the document and save the changes to the database.

Another difference is that create method can insert multiple documents at once, by passing an array of documents as parameter, while save is intended to be used on a single document.

So, if you want to create a new instance of a model and save it to the database in one step, you can use the create method. If you have an existing instance of a model that you want to save to the database, you should use the save method.

Also, if you have any validation or pre-save hook in your content schema, this will be triggered when using the create method.

You can either use `save()` or `create()`.

`save()` can only be used on a new document of the model while `create()` can be used on the model. Below, I have given a simple example.

Tour Model
----------

    const mongoose = require("mongoose");
    
    const tourSchema = new mongoose.Schema({
      name: {
        type: String,
        required: [true, "A tour must have a name"],
        unique: true,
      },
      rating: {
        type: Number,
        default:3.0,
      },
      price: {
        type: Number,
        required: [true, "A tour must have a price"],
      },
    });
    
    const Tour = mongoose.model("Tour", tourSchema);
    
    module.exports = Tour;
    

Tour Controller
---------------

    const Tour = require('../models/tourModel');
    
    exports.createTour = async (req, res) => {
      // method 1
      const newTour = await Tour.create(req.body);
    
      // method 2
      const newTour = new Tour(req.body);
      await newTour.save();
    }
    

Make sure to use either Method 1 or Method 2.

I'm quoting Mongoose's [Constructing Documents](https://mongoosejs.com/docs/models.html#constructing-documents) documentation:

    const Tank = mongoose.model('Tank', yourSchema);
    
    const small = new Tank({ size: 'small' });
    small.save(function (err) {
      if (err) return handleError(err);
      // saved!
    });
    
    // or
    
    Tank.create({ size: 'small' }, function (err, small) {
      if (err) return handleError(err);
      // saved!
    });
    
    // or, for inserting large batches of documents
    Tank.insertMany([{ size: 'small' }], function(err) {
    
    });

What is the diffrent between save vs insert vs create in mongoose?

在Mongoose中，`__v`属性是一个版本键，默认情况下会添加到MongoDB文档中，用于表示文档的版本。如果要在查询时从结果中排除这个属性，有多种方法可以做到。

### 1. 查询时明确排除`__v`属性

在执行查询时，可以通过在查询的`projection`参数中设置`__v: 0`来排除这个字段。

```javascript
Model.find({}, { __v: 0 }).then(result => {
    console.log(result);
});
```

### 2. 使用Schema的选项来全局隐藏`__v`

在定义Mongoose Schema时，可以设置`versionKey`为`false`，这样在该模型的所有实例中都会隐藏`__v`字段。

```javascript
const schema = new mongoose.Schema({ name: String }, { versionKey: false });
const Model = mongoose.model('Model', schema);

const doc = new Model({ name: 'Hide __v example' });
console.log(doc); // `__v` will not be part of the document
```

### 3. 使用虚拟属性来过滤`__v`

虚拟属性是Mongoose中一个非常灵活的特性，你可以定义一个虚拟属性来获取除去`__v`字段后的文档对象。

```javascript
schema.virtual('documentWithoutVersionKey').get(function() {
    const obj = this.toObject();
    delete obj.__v;
    return obj;
});
```

之后，你可以使用这个虚拟属性来获取没有`__v`属性的文档对象。

### 4. 使用`toJSON`和`toObject`转换选项

在Schema定义中，也可以通过设置`toJSON`和`toObject`的transform选项来排除`__v`。

```javascript
schema.set('toJSON', {
    transform: function(doc, ret, options) {
        delete ret.__v;
        return ret;
    }
});

schema.set('toObject', {
    transform: function(doc, ret, options) {
        delete ret.__v;
        return ret;
    }
});
```

这样在每次调用`toJSON()`或`toObject()`方法时，`__v`字段都会被自动排除。

### 实例

例如，假设我有一个用户模型，并且我不希望在任何API响应中返回`__v`字段，我可以在定义该用户Schema时设置`versionKey: false`：

```javascript
const userSchema = new mongoose.Schema({
    username: String,
    email: String
}, { versionKey: false });

const User = mongoose.model('User', userSchema);
```

这样，无论何时创建用户模型的实例，都不会包含`__v`字段。

总结来说，隐藏`__v`属性可以根据项目要求，在查询时动态排除，或者在模型定义时通过Schema选项进行全局设置。

You can disable the "\_\_v" attribute in your Schema definitions by setting the `versionKey` option to `false`. For example:

    var widgetSchema = new Schema({ ... attributes ... }, { versionKey: false });
    

I don't think you can globally disable them, but can only do it per Schema. You can read more about Schema's [options here](http://mongoosejs.com/docs/guide.html#versionKey). You might also find the [Schema set method](http://mongoosejs.com/docs/api.html#schema_Schema-set) helpful.

To disable '\_\_v' property, which is not recommended, use the [`versionKey` schema option](http://mongoosejs.com/docs/guide.html#versionKey):

    var Schema = new Schema({...}, { versionKey: false });
    

To hide it from all queries, which sometimes can be [not what you want too](https://stackoverflow.com/a/15025570/2153350), use the [`select` schema type option](http://mongoosejs.com/docs/api.html#schematype_SchemaType-select):

    var Schema = new Schema({ __v: { type: Number, select: false}})

Two ways:

1.  `{versionKey: false}`
    
2.  when you query, like `model.findById(id).select('-__v')`
    

`'-'` means exclude the field

define a `toObject.transform` function, and make sure you always call `toObject` when getting documents from mongoose.

    var SomeSchema = new Schema({
        <some schema spec>
      } , {
        toObject: {
          transform: function (doc, ret, game) {
            delete ret.__v;
          }
        }
    });

Try this it will remove \_v from every query response.

    // transform for sending as json
    function omitPrivate(doc, obj) {
        delete obj.__v;
        return obj;
    }
    
    // schema options
    var options = {
        toJSON: {
            transform: omitPrivate
        }
    };
    // schema
    var Schema = new Schema({...}, options);

How to hided __v property in mongoose?

在Mongoose中，要获取最新的文档记录或者最旧的文档记录，可以通过排序（Sorting）和限制返回结果数量来实现。

比如说，您有一个名为 `Article` 的模型，该模型表示一个博客文章的集合。每个文档都有一个 `createdAt` 字段，该字段会在创建新文档时自动设置为当前日期和时间。

获取最新的文档记录，您可以使用 `.findOne()` 方法结合 `.sort()` 方法，按照 `createdAt` 字段的降序排列，并限制结果为1：

```javascript
// 获取最新的文章记录
Article.findOne().sort('-createdAt').exec((err, latestDocument) => {
  if (err) {
    console.error(err);
  } else {
    console.log('最新的文档记录是:', latestDocument);
  }
});
```

这里，`'-createdAt'` 代表了按照 `createdAt` 字段的降序排列。如果您想获取最旧的文档记录，只需要将排序改为升序：

```javascript
// 获取最旧的文章记录
Article.findOne().sort('createdAt').exec((err, oldestDocument) => {
  if (err) {
    console.error(err);
  } else {
    console.log('最旧的文档记录是:', oldestDocument);
  }
});
```

这样，您就可以根据需要获取最新或最旧的文档记录。如果您的文档中没有 `createdAt` 或其他可以用于排序的时间戳字段，那么您需要添加一个时间戳字段或者使用其他字段进行排序。

Mongoose 3.x is complaining about the `[]` parameter in your [`findOne`](https://mongoosejs.com/docs/api/model.html#model_Model.findOne) calls as the array format is no longer supported for the parameter that selects the fields to include.

Try this instead to find the newest:

    Tweet.findOne({}, {}, { sort: { 'created_at' : -1 } }, function(err, post) {
      console.log( post );
    });
    

Change the `-1` to a `1` to find the oldest.

But because you're not using any field selection, it's somewhat cleaner to chain a couple calls together:

    Tweet.findOne().sort({created_at: -1}).exec(function(err, post) { ... });
    

Or even pass a string to [`sort`](http://mongoosejs.com/docs/api.html#query_Query-sort):

    Tweet.findOne().sort('-created_at').exec(function(err, post) { ... });

### Fast and Simple - One Line Solution

Get **`10 latest documents`**

    MySchema.find().sort({ _id: -1 }).limit(10)
    

Get **`10 oldest documents`**

    MySchema.find().sort({ _id: 1 }).limit(10)
    

In case you want sorting based on some other property i.e. `createdAt` and get the oldest or latest. It is similar to the above query.

    MySchema.find().sort({ createdAt: -1 }).limit(10)  // 10 latest docs
    MySchema.find().sort({ createdAt: 1 }).limit(10) // 10 oldest docs

for version ~3.8 mongoose

to find the last entry

    model.findOne().sort({ field: 'asc', _id: -1 }).limit(1)
    

or using

    model.findOne().sort({ field: -_id }).limit(1)

collectionName.findOne().sort({$natural: -1}).limit(1).exec(function(err, res){
        if(err){
            console.log(err);
        }
        else{
            console.log(res);
        }
    }
    

This will give you the last document recorded on the database. Just follow the same concept.

await Model.find().sort({$natural:-1}).limit(1); //for the latest record
    await Model.find().sort({$natural:1}).limit(1); //for the oldest record
    

This one works for me. using mongodb natural order [https://docs.mongodb.com/manual/reference/operator/meta/natural/](https://docs.mongodb.com/manual/reference/operator/meta/natural/)

How to get the latest and oldest record in mongoose js?

在 Mongoose 中，如果您想要将模型中的某个字段设置为 `ObjectId` 数据类型，通常是为了创建一个引用其他MongoDB文档的字段。您可以使用 `Schema.Types.ObjectId` 来指定字段类型。这通常与 `ref` 选项结合使用，以指明该 `ObjectId` 引用的模型。

以下是一个如何在Mongoose模型中定义 `ObjectId` 字段的例子：

```javascript
const mongoose = require('mongoose');
const { Schema } = mongoose;

// 定义一个新的Schema
const userSchema = new Schema({
  // ... 其他字段
  // 这里定义了一个名为`profile`的字段，它引用了`Profile`模型
  profile: {
    type: Schema.Types.ObjectId,
    ref: 'Profile'
  }
});

// 创建一个名为`User`的模型
const User = mongoose.model('User', userSchema);
```

在上述例子中，`profile` 字段被设置为 `ObjectId` 类型，并且通过 `ref` 属性与 `Profile` 模型相关联。这意味着 `profile` 字段应该包含一个存储在 `Profile` 集合中的文档的ID。

当您在使用Mongoose查询包含 `ObjectId` 引用的文档时，可以使用 `.populate()` 方法来自动替换这些字段的 `ObjectId` 为关联文档的数据。例如：

```javascript
User.findById(userId)
  .populate('profile') // 这会获取 `profile` 字段引用的 `Profile` 文档
  .exec((err, user) => {
    if (err) {
      // 处理错误
    } else {
      // 这里的 `user.profile` 将是一个完整的 `Profile` 文档而不只是一个ID
      console.log(user.profile);
    }
  });
```

这个 `.populate()` 调用告诉Mongoose，在查询完成时填充 `profile` 字段，而不仅仅是返回 `ObjectId`。这样，您就可以访问关联的 `Profile` 文档的所有信息。

Unlike traditional RBDMs, mongoDB doesn't allow you to define any random field as the primary key, the \_id field MUST exist for all standard documents.

For this reason, it doesn't make sense to create a separate uuid field.

In mongoose, the ObjectId type is used not to create a new uuid, rather it is mostly used to reference other documents.

Here is an example:

    var mongoose = require('mongoose');
    
    var Schema = mongoose.Schema,
        ObjectId = Schema.ObjectId;
    var Schema_Product = new Schema({
        categoryId  : ObjectId, // a product references a category _id with type ObjectId
        title       : String,
        price       : Number
    });
    

As you can see, it wouldn't make much sense to populate categoryId with a ObjectId.

However, if you do want a nicely named uuid field, mongoose provides virtual properties that allow you to proxy (reference) a field.

Check it out:

    var mongoose = require('mongoose');
    
    var Schema = mongoose.Schema,
        ObjectId = Schema.ObjectId;
    var Schema_Category = new Schema({
        title       : String,
        sortIndex   : String
    });
    
    Schema_Category.virtual('categoryId').get(function() {
        return this._id;
    });
    

So now, whenever you call category.categoryId, mongoose just returns the \_id instead.

You can also create a "set" method so that you can set virtual properties, check out [this link](http://mongoosejs.com/docs/virtuals.html) for more info

I was looking for a different answer for the question title, so maybe other people will be too.

To set type as an ObjectId (so you may reference `author` as the author of `book`, for example), you may do like:

    const Book = mongoose.model('Book', {
      author: {
        type: mongoose.Schema.Types.ObjectId, // here you set the author ID
                                              // from the Author colection, 
                                              // so you can reference it
        required: true
      },
      title: {
        type: String,
        required: true
      }
    });

My solution on using ObjectId

    // usermodel.js
    
    const mongoose = require('mongoose')
    const Schema = mongoose.Schema
    const ObjectId = Schema.Types.ObjectId
    
    
    let UserSchema = new Schema({
       username: {
         type: String
       },
       events: [{
         type: ObjectId,
         ref: 'Event' // Reference to some EventSchema
       }]
    })
    
    UserSchema.set('autoIndex', true)
    
    module.exports = mongoose.model('User', UserSchema)
    

Using mongoose's [populate](http://mongoosejs.com/docs/populate.html) method

    // controller.js
    
    const mongoose = require('mongoose')
    const User = require('./usermodel.js')
    
    let query = User.findOne({ name: "Person" })
    
    query.exec((err, user) => {
      if (err) {
         console.log(err)
      }
    
      user.events = events
      // user.events is now an array of events
    })

The solution provided by @dex worked for me. But I want to add something else that also worked for me: Use

    let UserSchema = new Schema({
       username: {
         type: String
       },
       events: [{
         type: ObjectId,
         ref: 'Event' // Reference to some EventSchema
       }]
    })
    

if what you want to create is an Array reference. But if what you want is an Object reference, which is what I think you might be looking for anyway, remove the brackets from the _value_ prop, like this:

    let UserSchema = new Schema({
       username: {
         type: String
       },
       events: {
         type: ObjectId,
         ref: 'Event' // Reference to some EventSchema
       }
    })
    

Look at the 2 snippets well. In the second case, the value prop of key events does not have brackets over the object def.

You can directly define the ObjectId

var Schema = new mongoose.Schema({ categoryId : mongoose.Schema.Types.ObjectId, title : String, sortIndex : String })

Note: You need to import the mongoose module

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