Using populate on schema with combined Map of Array<Schema> will not return Array in v6

[lattam]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.6.3

Node.js version

14.16.1

MongoDB server version

4.4.3

Description

Using populate on schema with "Map of Array" will ignore the Array part and will return only one Schema object after upgrading from mongoose 5 to 6.

Steps to Reproduce

const mongoose = require('mongoose');

const connection = mongoose.createConnection('mongodb://localhost:27037/test');

const User = new mongoose.Schema({
  name: String,
  addresses: {
    type: Map,
    of: [{
      type: mongoose.Schema.Types.ObjectId,
      ref:  'Address',
    }],
    default: {},
  },
});

const Address = new mongoose.Schema({
  city: String,
});

const UserModel = connection.model('User', User);
const AddressModel = connection.model('Address', Address);

async function init() {
  await UserModel.deleteMany({});
  await AddressModel.deleteMany({});

  const address = await new AddressModel({ city: 'London' }).save();

  await new UserModel({
    name: 'Name',
    addresses: {
      home: [address._id]
    }
  }).save();
}

async function test() {
  const query = UserModel.find({});

  query.populate({
    path: 'addresses.home',
  });

  const documents = await query.exec();

  console.log(JSON.stringify(documents));
  // v5.X.X outputs:
  // [{"_id":"63383ac599e8f5243db0cc8f","name":"Name", "addresses":{"home":[{"_id":"63383ac599e8f5243db0cc8d","city":"London"}]}}]

  // v6.X.X outputs:
  // [{"_id":"63383b503110bec5287a2ed7","name":"Name","addresses":{"home":{"_id":"63383b503110bec5287a2ed5","city":"London"}}}]
}

async function run() {
  await init();
  await test();
  connection.close();
}

run();

Expected Behavior

Return the Array of sub elements, not just the first object.

[chinmaykumbhare]

hi @lattam can you assign me this issue? I can fix it.

[IslandRhythms]

const mongoose = require('mongoose');

const connection = mongoose.createConnection('mongodb://localhost:27017/test');

const User = new mongoose.Schema({
  name: String,
  addresses: {
    type: Map,
    of: [{
      type: mongoose.Schema.Types.ObjectId,
      ref:  'Address',
    }],
    default: {},
  },
});

const Address = new mongoose.Schema({
  city: String,
});

const UserModel = connection.model('User', User);
const AddressModel = connection.model('Address', Address);

async function init() {
  await UserModel.deleteMany({});
  await AddressModel.deleteMany({});

  const address = await new AddressModel({ city: 'London' }).save();

  await new UserModel({
    name: 'Name',
    addresses: {
      home: [address._id]
    }
  }).save();
}

async function test() {
  const query = UserModel.find({});

  query.populate({
    path: 'addresses.home',
  });

  const documents = await query.exec();

  console.log(JSON.stringify(documents));
  // v5.X.X outputs:
  // [{"_id":"63383ac599e8f5243db0cc8f","name":"Name", "addresses":{"home":[{"_id":"63383ac599e8f5243db0cc8d","city":"London"}]}}]

  // v6.X.X outputs:
  // [{"_id":"63383b503110bec5287a2ed7","name":"Name","addresses":{"home":{"_id":"63383b503110bec5287a2ed5","city":"London"}}}]
}

async function run() {
  await init();
  await test();
  connection.close();
}

run();