What is the " __v " field in Mongoose

The __v field in Mongoose is a hidden field representing the version number of the document. It is used internally by Mongoose to support version control for documents, employing an optimistic locking mechanism to prevent conflicts during concurrent modifications. Whenever a document is created or saved in Mongoose, if the document contains array fields and operations modifying the array (such as adding or removing elements) are performed, Mongoose automatically updates the value of the __v field. This allows Mongoose to check the value of the __v field when multiple users or processes attempt to modify the same document concurrently, determining whether the document has been modified by others and thus deciding if the current modification should proceed. This version number is automatically maintained, and developers typically do not need to modify it manually. Of course, if you don't need this field, you can disable it when defining the Mongoose Schema using the option { versionKey: false }. For example, if we define a simple user model and wish to disable the __v field, we can do the following:

javascript
const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const userSchema = new Schema({
  name: String,
  email: String,
  password: String
}, { versionKey: false }); // Disable __v field

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

However, in most cases, developers retain this field to leverage its concurrency control functionality.

To disable the __v field in NestJS, you need to add the option to the Schema() decorator.

typescript
@Schema({ versionKey: false })