Nested populate doesn't work when the main property has a default: null definition

[Ncifra]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
When trying to do a nested populate from the document's populate() method, nothing happens

If the current behavior is a bug, please provide the steps to reproduce.

const assert = require('assert');
const mongoose = require('mongoose');
mongoose.set('debug', true);

const GITHUB_ISSUE = `gh8518`;
const connectionString = `mongodb://localhost:27017/${GITHUB_ISSUE}`;
const {Schema} = mongoose;

run().then(() => console.log('done')).catch(error => console.error(error.stack));

async function run() {
    const db = await mongoose.connect(connectionString);
    await mongoose.connection.dropDatabase();

    const workingSchema = mongoose.Schema({
        userApproval: {
            users: [
                {
                    user: {
                        type: mongoose.Schema.Types.ObjectId,
                        ref: 'Populated'
                    }
                }
            ]
        }
    });

    const failingSchema = mongoose.Schema({
        userApproval: {
            type: {
                users: [
                    {
                        user: {
                            type: mongoose.Schema.Types.ObjectId,
                            ref: 'Populated'
                        }
                    }
                ]
            },
            default: null
        }
    });

    const schemaToPopulate = mongoose.Schema({
        test: {
            type: String,
            default: "TEST"
        }
    });

    const W = db.model('Test', workingSchema);
    const F = db.model('TestFail', failingSchema);
    const P = db.model('Populated', schemaToPopulate);

    let p = await P.create({});
    let w = await W.create({userApproval: {users: [{user: p._id}]}});
    let f = await F.create({userApproval: {users: [{user: p._id}]}});
    await w.populate('userApproval.users.user', 'test').execPopulate();
    await f.populate('userApproval.users.user', 'test').execPopulate();
    console.log(w.userApproval.users);
    console.log(f.userApproval.users);
}

What is the expected behavior?
Even with a default: null for a property that is an object, the nested populate should still work.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node 10.x
Mongoose 5.8.7 (.8.8 doesn't seem to be in the NPM repo) 5.8.9
MongoDB 4.0.9

[vkarpov15]

Your schema definition is incorrect. As written, your failingSchema defines userApproval as a mixed path, so the ref underneath the Mixed gets ignored. Do this instead:

    const failingSchema = mongoose.Schema({
        userApproval: {
            type: mongoose.Schema({
                users: [
                    {
                        user: {
                            type: mongoose.Schema.Types.ObjectId,
                            ref: 'Populated'
                        }
                    }
                ]
            }),
            default: null
        }
    });

More evidence for why we need something like #7181