Virtual Population of Field on a Nested Embedded Discriminator Not Working

[jimmytsao]

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

What is the current behavior?
When trying to perform a virtual population of a field on an embedded discriminator that is at the root of the model, everything is populated correctly. However, when I try do the same on a nested embedded discriminator, the results are not populated. See working and non-working example in repro script section.

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

Here's are some examples:

Virtual Population WORKING When Embedded Discriminator Field Is At The Root

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({ events: [eventSchema] });  // <-- Embedded Discrminator at root
  const docArray = batchSchema.path('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 = {
    events: [
      { kind: 'Clicked', element: '#hero', message: 'hello', users: [1] },
      { kind: 'Purchased', product: 'action-figure-1', message: 'world' }
    ]
  };

  Promise.all([UserModel.create(user), Batch.create(batch)])
    .then(function(){
      Batch.find({})
        // Populate virtual field of embedded discriminator
        .populate('events.users_$')
        .lean()
        .then(results => console.log(results) )
    })
 
  /*  Correct Results with "events.users_$" populated
  [
      {
          "_id": "5ae8ccc0daad6931954ceebf",
          "events": [
              {
                  "users": [
                      1
                  ],
                  "_id": "5ae8ccc0daad6931954ceec1",
                  "kind": "Clicked",
                  "element": "#hero",
                  "message": "hello",
                  "users_$": [
                      {
                          "_id": "5ae8ccc0daad6931954ceebe",
                          "employeeId": 1,
                          "name": "Test name",
                          "__v": 0
                      }
                  ]
              },
              {
                  "_id": "5ae8ccc0daad6931954ceec0",
                  "kind": "Purchased",
                  "product": "action-figure-1",
                  "message": "world"
              }
          ],
          "__v": 0
      }
  ]
  */
})

Virtual Population NOT WORKING When Embedded Discriminator Field Is Nested

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: { // <--- Added Nesting
      events: [eventSchema]
    }
  });

  const docArray = batchSchema.path('nested.events'); // <--- Added Nesting

  // 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: {   // <--- Added Nesting
      events: [
        { kind: 'Clicked', element: '#hero', message: 'hello', users: [1] },
        { kind: 'Purchased', product: 'action-figure-1', message: 'world' }
      ]
    }
  };

  Promise.all([UserModel.create(user), Batch.create(batch)])
    .then(function(){
      Batch.find({})
        // Populate virtual field of embedded discriminator
        .populate('nested.events.users_$') // <--- Added Nesting
        .lean()
        .then(results => console.log(results) )
    })
  }
)
  /*  Results. Expected "nested.events.users_$" to be populated
  [
    {
        "_id": "5ae8d2f453e7c03deb827438",
        "nested": {
            "events": [
                {
                    "users": [
                        1
                    ],

                    "_id": "5ae8d2f453e7c03deb82743a",
                    "kind": "Clicked",
                    "element": "#hero",
                    "message": "hello"
                },
                {
                    "_id": "5ae8d2f453e7c03deb827439",
                    "kind": "Purchased",
                    "product": "action-figure-1",
                    "message": "world"
                }
            ]
        },
        "__v": 0
    }
  ]
  */

What is the expected behavior?
The virtual field at "nested.events.users_$" is populated

Please mention your node.js, mongoose and MongoDB version.
node: 10.0.0
mongoose: 5.0.17
mongodb: 3.6.4

[vkarpov15]

Thanks for reporting, will fix ASAP 👍