Error with Virtual Population of Field on Embedded Discriminator

[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, I receive the following error:

VM182 utils.js:214 Uncaught TypeError: Cannot read property 'length' of undefined
    at getModelsMapForPopulate (/Users/Documents/Code/mongoose-test/node_modules/mongoose/lib/model.js:3462:24)
    at populate (/Users/Documents/Code/mongoose-test/node_modules/mongoose/lib/model.js:3039:15)
    at _populate (/Users/Documents/Code/mongoose-test/node_modules/mongoose/lib/model.js:3007:5)
    at utils.promiseOrCallback.cb (/Users/Documents/Code/mongoose-test/node_modules/mongoose/lib/model.js:2980:5)
    at Object.promiseOrCallback (/Users/Documents/Code/mongoose-test/node_modules/mongoose/lib/utils.js:211:14)
    at Function.Model.populate (/Users/Documents/Code/mongoose-test/node_modules/mongoose/lib/model.js:2979:16)
    at cb (/Users/Documents/Code/mongoose-test/node_modules/mongoose/lib/query.js:1325:17)
    at result (/Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/utils.js:413:17)
    at executeCallback (/Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/utils.js:405:9)
    at handleCallback (/Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/utils.js:128:55)
    at self.close (/Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/cursor.js:934:60)
    at handleCallback (/Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/utils.js:128:55)
    at completeClose (/Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/cursor.js:1073:14)
    at Cursor.close (/Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/cursor.js:1086:10)
    at /Users/Documents/Code/mongoose-test/node_modules/mongodb/lib/cursor.js:934:21
    at handleCallback (/Users/Documents/Code/mongoose-test/node_modules/mongodb-core/lib/cursor.js:178:5)

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

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] });
  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) )
    })
})

What is the expected behavior?
The query should not have errored and should have populated the virtual field.

Please mention your node.js, mongoose and MongoDB version.
node: v9.7.1, mongoose: 5.0.11, MongoDB: v3.2.5

[sobafuchs]

youre right, this looks like a confirmed bug, thank you for the full repro script