Mongoose - query to get data from multiple collections

When interacting with MongoDB using Mongoose, there are several methods to retrieve data from multiple collections. Mongoose is an object modeling tool for MongoDB built on Node.js, providing mechanisms for querying across collections. Here are some common methods:

1. Populate

Mongoose's populate method allows you to automatically replace specified paths in documents with documents from another collection during a query. It is based on MongoDB's $lookup aggregation operation and can be used to address relational data requirements. Here is an example:

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

// Assume two models: User and Order

const UserSchema = new Schema({
  name: String,
  // Other fields
});

const OrderSchema = new Schema({
  product: String,
  quantity: Number,
  user: {
    type: Schema.Types.ObjectId,
    ref: 'User' // This specifies the reference to the User model
  },
  // Other fields
});

const User = mongoose.model('User', UserSchema);
const Order = mongoose.model('Order', OrderSchema);

// When retrieving order information along with associated user information:

Order.find().populate('user').exec((err, orders) => {
  if (err) throw err;
  console.log(orders); // orders now include detailed user information
});

2. Aggregation

Mongoose's aggregation framework can perform more complex queries, such as retrieving data from multiple collections. Here is an example using an aggregation pipeline:

javascript
const Order = mongoose.model('Order', OrderSchema);

Order.aggregate([
  {
    $lookup: {
      from: 'users', // Note: this is the MongoDB collection name, typically in lowercase plural form
      localField: 'user', // The associated field in the Order collection
      foreignField: '_id', // The associated field in the User collection
      as: 'userDetails' // Alias for the result
    }
  }
]).exec((err, result) => {
  if (err) throw err;
  console.log(result); // Each order item now includes a userDetails array with associated user information.
});

3. Manual References and Queries

In some cases, if you want finer control over the query process, you might choose not to use populate and instead store the _id of the referenced document, then manually execute multiple queries. For example:

javascript
// First, query the Order collection
Order.find().exec((err, orders) => {
  if (err) throw err;

  // Then, for each order, query user information based on the stored user ID
  const userIds = orders.map(order => order.user);
  User.find({ _id: { $in: userIds } }).exec((userErr, users) => {
    if (userErr) throw userErr;

    // You may need to manually combine order and user information here
    const combinedData = orders.map(order => {
      return {
        ...order.toObject(),
        user: users.find(u => u._id.equals(order.user)),
      };
    });

    console.log(combinedData);
  });
});

Each method has its use cases. populate is better suited for simple relational data queries, while the aggregation framework is used for more complex queries. Manual reference queries are used when you need more control. In actual development, the choice depends on specific requirements and performance considerations.