How to set _id to db document in Mongoose?

By default, each document in a Mongoose model has an _id field, which is typically automatically generated as a MongoDB ObjectId. A MongoDB ObjectId is a 12-byte field that typically contains a timestamp, a random value, and an increment counter to ensure the uniqueness of each _id.

However, there may be specific requirements where you need to set _id to a specific value or use different data types (such as strings or numbers). In Mongoose, you can customize the type and value of _id when defining the Schema.

Here is an example of how to customize _id in Mongoose:

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

// Define a new Schema where the _id field is explicitly set to String type
const customIdSchema = new Schema({
  _id: { type: String, required: true },
  name: String,
  age: Number
});

// Create the model
const User = mongoose.model('User', customIdSchema);

// Create a new document instance with a custom _id
const newUser = new User({
  _id: 'custom-id-12345',
  name: 'Zhang San',
  age: 30
});

// Save to the database
newUser.save()
  .then(doc => {
    console.log('Document saved successfully', doc);
  })
  .catch(err => {
    console.error('Error saving document', err);
  });

In this example, we define a User model with _id explicitly set to String type. When creating the newUser instance, we manually specify _id as 'custom-id-12345'. This ensures that MongoDB uses the custom ID instead of generating a default ObjectId.

This approach allows you to customize the ID based on business needs, such as using a user's email address or other business identifiers as the document's unique identifier. However, when manually setting _id, you must ensure its uniqueness to avoid errors from duplicate _id values.