Virtual Population of Field on a Nested Embedded Discriminator Not Working When Being Used in Models With Nested Schemas

[jimmytsao]

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

What is the current behavior?
When I try to populate an embedded discriminator from a model that has a nested schema defined by an Object literal everything works. However, if I do the same but use a nested schema instantiated from mongoose.Schema, it no longer works . See working and non-working example in repro script section.

This may be related to: #6411 and #6488

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

Here's are some examples:

Virtual Population WORKING When Nested Schema is Defined By An Object Literal

const mongoose = require('mongoose')
mongoose.connect('mongodb://localhost/mongoosetest')
mongoose.connection.once('open', () => {

  const Schema = mongoose.Schema

  // Generate Users Model
  const userSchema = new Schema({ employeeId: Number, name: String })
  const UserModel = mongoose.model('Users', userSchema, 'users')

  // Generate Embedded Discriminators
  const eventSchema = new Schema(
    { message: String },
    { discriminatorKey: 'kind'}
  );

  const batchSchema = new Schema({
    nested: { 
      events: [eventSchema]
    }
  });

  const docArray = batchSchema.path('nested.events'); 

  // First embedded discriminator schema
  const clickedSchema = new Schema(
    {
      element: { type: String },
      users: [ Number ]
    },
    {
      toJSON: { virtuals: true},
      toObject: { virtuals: true}
    }
  );

  // Add virtual to first embedded discriminator schema for virtual population
  clickedSchema.virtual('users_$', {
    ref: 'Users',
    localField: 'users',
    foreignField: 'employeeId'
  })
  
  const Clicked = docArray.discriminator('Clicked', clickedSchema);

  // Second embedded discriminator
  const Purchased = docArray.discriminator('Purchased', new Schema({
    product: { type: String }
  }));

  const Batch = mongoose.model('EventBatch', batchSchema);

  // Generate Items
  const user = { employeeId: 1, name: 'Test name' }
  const batch = {
    nested: {   
      events: [
        { kind: 'Clicked', element: '#hero', message: 'hello', users: [1] },
        { kind: 'Purchased', product: 'action-figure-1', message: 'world' }
      ]
    }
  };

  // Clearing out the collections before creating items
  Promise.all([ UserModel.remove({}), Batch.remove({}) ])
  .then(() => Promise.all([UserModel.create(user), Batch.create(batch)]))
  .then(function(){
    Batch.find({})
      // Populate virtual field of embedded discriminator
      .populate('nested.events.users_$') 
      .lean()
      .then(results => {
        console.log(JSON.stringify(results, null, 2))
      })
  })
})

Received correct output with "users_$" populated

[
  {
    "_id": "5b01e27d2956fa8bcbd4a5f9",
    "nested": {
      "events": [
        {
          "users": [
            1
          ],
          "_id": "5b01e27d2956fa8bcbd4a5fb",
          "kind": "Clicked",
          "element": "#hero",
          "message": "hello",
          "users_$": [
            {
              "_id": "5b01e27d2956fa8bcbd4a5f8",
              "employeeId": 1,
              "name": "Test name",
              "__v": 0
            }
          ]
        },
        {
          "_id": "5b01e27d2956fa8bcbd4a5fa",
          "kind": "Purchased",
          "product": "action-figure-1",
          "message": "world",
          "users_$": []
        }
      ]
    },
    "__v": 0
  }
]

Virtual Population NOT WORKING When Nested Schema is by Schema instantiated from mongoose.Schema

const mongoose = require('mongoose')
mongoose.connect('mongodb://localhost/mongoosetest')
mongoose.connection.once('open', () => {

  const Schema = mongoose.Schema

  // Generate Users Model
  const userSchema = new Schema({ employeeId: Number, name: String })
  const UserModel = mongoose.model('Users', userSchema, 'users')

  // Generate Embedded Discriminators
  const eventSchema = new Schema(
    { message: String },
    { discriminatorKey: 'kind'}
  );

  const batchSchema = new Schema({
    nested: new Schema({   //<== Schema from mongoose.Schema instead of object literal
      events: [eventSchema]
    })
  });

  const docArray = batchSchema.path('nested.events'); 

  // First embedded discriminator schema
  const clickedSchema = new Schema(
    {
      element: { type: String },
      users: [ Number ]
    },
    {
      toJSON: { virtuals: true},
      toObject: { virtuals: true}
    }
  );

  // Add virtual to first embedded discriminator schema for virtual population
  clickedSchema.virtual('users_$', {
    ref: 'Users',
    localField: 'users',
    foreignField: 'employeeId'
  })
  
  const Clicked = docArray.discriminator('Clicked', clickedSchema);

  // Second embedded discriminator
  const Purchased = docArray.discriminator('Purchased', new Schema({
    product: { type: String }
  }));

  const Batch = mongoose.model('EventBatch', batchSchema);

  // Generate Items
  const user = { employeeId: 1, name: 'Test name' }
  const batch = {
    nested: {   
      events: [
        { kind: 'Clicked', element: '#hero', message: 'hello', users: [1] },
        { kind: 'Purchased', product: 'action-figure-1', message: 'world' }
      ]
    }
  };

  // Clearing out the collections before creating items
  Promise.all([ UserModel.remove({}), Batch.remove({}) ])
  .then(() => Promise.all([UserModel.create(user), Batch.create(batch)]))
  .then(function(){
    Batch.find({})
      // Populate virtual field of embedded discriminator
      .populate('nested.events.users_$') 
      .lean()
      .then(results => {
        console.log(JSON.stringify(results, null, 2))
      })
  })
})

Received incorrect output with "users_$" not populated

[
  {
    "_id": "5b01e42b623c3a8cf6bf256e",
    "nested": {
      "_id": "5b01e42b623c3a8cf6bf256f",
      "events": [
        {
          "users": [
            1
          ],
          "_id": "5b01e42b623c3a8cf6bf2571",
          "kind": "Clicked",
          "element": "#hero",
          "message": "hello"
        },
        {
          "_id": "5b01e42b623c3a8cf6bf2570",
          "kind": "Purchased",
          "product": "action-figure-1",
          "message": "world"
        }
      ]
    },
    "__v": 0
  }
]

What is the expected behavior?
Expected "users_$" to be populated

Please mention your node.js, mongoose and MongoDB version.
node: v8.11.1
mongoose: 5.1.1
mongo: 3.6.4

[vkarpov15]

Thanks for your continued detailed bug reports, I appreciate your help. Will fix this ASAP 👍