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
      }
    ]
  }
]