How to get reference collection data in mongoose using lookup?

Lookup:

const course = await mongoose.model('Course').aggregate()
    .match({ _id: new mongoose.Types.ObjectId(id) })
    .lookup({
        from: 'User',
        foreignField: '_id',
        localField: 'userId',
        as: 'user'
    })

Course schema:

const CourseSchema = new Schema({
  title: String,
  userId: {
    type: mongoose.Types.ObjectId,
    ref: 'User'
  },
}, { strict: false });

Expected output:

{
    "course": [
        {
            "_id": "64b000c085e9b1aec5975249",
            "title": "test",
            "userId": "64affc47d81a4bfba92b5540",
            "user": [], // data here should be present but empty why?
        }
    ]
}

How do I get user data in the above output?

Works using populate:

const course = await mongoose.model('Course')
            .find({ _id: new mongoose.Types.ObjectId(id) })
            .populate('userId').exec();

But how to achieve this using aggregation pipeline?

The collection name is users, not User. See option: collection

> Mongoose by default produces a collection name by passing the model name to the utils.toCollectionName method. This method pluralizes the name.

Let's see the utils.toCollectionName method:

exports.toCollectionName = function(name, pluralize) {
  if (name === 'system.profile') {
    return name;
  }
  if (name === 'system.indexes') {
    return name;
  }
  if (typeof pluralize === 'function') {
    return pluralize(name);
  }
  return name;
};

function pluralize(str) {
  let found;
  str = str.toLowerCase();
  if (!~uncountables.indexOf(str)) {
    found = rules.filter(function(rule) {
      return str.match(rule[0]);
    });
    if (found[0]) {
      return str.replace(found[0][0], found[0][1]);
    }
  }
  return str;
}

See full source code

As you can see, the model name 'User' will be converted to the collection name "users". So it should be from: "users", not from: "User" in lookup() method.

The localField and foreignField are wrong. The localField is the field in the local collection which is cources. The foreignField is the field in the foreign collection which is users.

• from: <foreign collection>
• localField: <field from local collection's documents>,
• foreignField: <field from foreign collection's documents>

A working example:

import mongoose from 'mongoose';
import util from 'util';
import { config } from '../../config';

mongoose.set('debug', true);
console.log(mongoose.version);

const UserSchema = new mongoose.Schema({ name: String });
const User = mongoose.model('User', UserSchema);

const CourseSchema = new mongoose.Schema(
	{
		title: String,
		userId: {
			type: mongoose.Types.ObjectId,
			ref: 'User',
		},
	},
	{ strict: false },
);
const Course = mongoose.model('Course', CourseSchema);

(async function main() {
	try {
		await mongoose.connect(config.MONGODB_URI);
		// seed
		const [u1] = await User.create([{ name: 'user-a' }]);
		const [c] = await Course.create([{ title: 'course a', userId: u1 }]);

		const r = await Course.aggregate()
			.match({ _id: c?._id })
			.lookup({
				from: 'users',
				localField: 'userId',
				foreignField: '_id',
				as: 'user',
			});

		console.log(util.inspect(r, false, null));
	} catch (error) {
		console.error(error);
	} finally {
		await mongoose.connection.close();
	}
})();

Logs:

7.3.2
Mongoose: users.insertOne({ name: 'user-a', _id: ObjectId("64b0e2768c75418f819995af"), __v: 0 }, {})
Mongoose: courses.insertOne({ title: 'course a', userId: ObjectId("64b0e2768c75418f819995af"), _id: ObjectId("64b0e2778c75418f819995b1"), __v: 0}, {})
Mongoose: courses.aggregate([ { '$match': { _id: new ObjectId("64b0e2778c75418f819995b1") } }, { '$lookup': { from: 'users', localField: 'userId', foreignField: '_id', as: 'user' } }], {})
[
  {
    _id: new ObjectId("64b0e2778c75418f819995b1"),
    title: 'course a',
    userId: new ObjectId("64b0e2768c75418f819995af"),
    __v: 0,
    user: [
      {
        _id: new ObjectId("64b0e2768c75418f819995af"),
        name: 'user-a',
        __v: 0
      }
    ]
  }
]

If your userId in Course collection is objectId instead of plain String
you can aggregate result by following query,

db.Course.aggregate([{$match:{_id:ObjectId("yourId")}}
,{$lookup:{
from: 'User',
localField: 'userId',
foreignField: "_id",
as: 'users'
}}])

if you have userId as plain String you need to use $addFields to convert your string id to ObjectId.

db.Course.aggregate([{$match:{_id:ObjectId("yourId")}}
,{$addFields:{'userId':{"$toObjectId":"$userId"}}}
,{$lookup:{
from: 'User',
localField: 'userId',
foreignField: "_id",
as: 'users'
}}])