Population fails when using "type"

[simllll]

I want to report a bug.

What is the current behavior?
When using a model that has a document defintion with "type", mongoose cannot populate the document correctly.

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

const mongoose = require('mongoose');

console.log(mongoose.version);

run().catch(error => console.error(error.stack));

async function run() {
    await mongoose.connect('mongodb://localhost:27017/test2');

    // Broken - population fails with "Null", and it tries to populate via "ModelA" (NOT B)
    const ModelA = mongoose.model('ModelA', new mongoose.Schema({
        bla: {
            select: false,
            type: {
                _reference: {
                    type: mongoose.Schema.Types.ObjectId,
                    ref: 'ModelB'
                },
            }
        }
    }));

    // here everything works - same model, only difference is that "defintion" is not done via "custom type".
    /* const ModelA = mongoose.model('ModelA', new mongoose.Schema({
        bla: {
            _reference: {
                type: mongoose.Schema.Types.ObjectId,
                ref: 'ModelB'
            }
        }
    })); */

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

    const b = new ModelB({name: 'test'});

    await b.save();

    const a = new ModelA({bla: {_reference: b._id}});
    await a.save();

    const result = await ModelA.find({}, '+bla')
        .populate('bla._reference');

    console.log('result', JSON.stringify(result, false, 3));
}

This results into:

{
      "_id": "5b9eb13f0001254b2a2ea3f0",
      "bla": {
         "_reference": null <-- NULL
      },
      "__v": 0
   }

What is the expected behavior?
Mongoose should also be able to populate documents when they are defined via "type".

Please mention your node.js, mongoose and MongoDB version.
mongoose 5.2.15, v8.11.4, mongodb 3.6

[vkarpov15]

Mongoose doesn't support the syntax you're using. In the above example, Mongoose assumes that bla is mixed type. You need to define a separate schema for _reference, like type: new Schema({ _reference: ... })