What does populate in mongoose mean

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

shell
Story.findOne({ title: Nintendo })

will return a Story of this kind:

shell
{
  _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.

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

will return

shell
{
  _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.

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

will return

shell
{
  _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

shell
_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 or https://mongoosejs.com/docs/populate.html

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

EXAMPLE without .populate()

shell
User.findOne({ name: Bob })

Will return

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

EXAMPLE with .populate()

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

Will return

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

This answer 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:

shell
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' }
  });