.populate() does not work from a base schema, using discriminators, when the related collection is defined in the polymorphic child.

[ericsaboia]

mongoose version: "^3.8.19"
related to: #1647

When you use discriminators, and want to query from the base schema, populating some related collection, which was defined in the polymorphic child schema, the query runs, but the result goes without the related population objects.

Example:

var mongoose = require('mongoose')
  , Schema = mongoose.Schema
  , util = require('util')
;

function BaseSchema() {
  Schema.apply(this, arguments);
  this.add( { name: String } );
}

util.inherits(BaseSchema, Schema);

var EventSchema = new BaseSchema();

var ConversionEvent = new BaseSchema({
  owner: { type: Schema.Types.ObjectId, ref: 'User', required: true }
});

var Base = mongoose.model('Event', EventSchema),
var Conversion = Base.discriminator('Conversion', ConversionEvent);

Conversion.create({
  name: 'test',
  owner: user._id
}, function (err, conversion) {

  Base.find().populate('owner').exec(function (err, bases) {
    // Owner object isen't attached
    console.log(bases); 
  });

});

[vkarpov15]

Hmm can you let me know what EventSchema looks like? Makes it much easier for me when I can just run your repro example as is.

[ericsaboia]

It's just an instance of the BaseSchema, I've updated the example.

[vkarpov15]

@ericsaboia this is by design. The schema underlying Base doesn't have an owner field, so .populate('owner') is invalid. Conversion.find().populate('owner') works as intended.

[ericsaboia]

Are you sure? because even when you query by the Base model, the returned object it's a instance of its child, and contains all the child fields:

For example:

Base.find({}, function(err, events) {
    expect.ok(events[0] instanceof Conversion);       // true
   console.log(events[0].owner) // Owner ObjectId
});

So in my opinion it's worth the implementation.

[vkarpov15]

True, but when you call populate() it explicitly uses the base model and so doesn't know about 'owner'. It's a quirk, but one that makes sense IMO. What's your use case?

[ericsaboia]

It's a feed on a social app. My base model it's called Activity, and then I have a lot of specialised types of Activity (checkin, post, promotion, and so on)

Some of my Activity's children have specific relations, so it's not declared at the base model (Activity).

At my feed service, I need to return a list of all kind of activities, with the populates necessary to fill the client.

Ps: the result is cached, that's why I decided to keep the call to "populate" instead of copy the necessary fields from the related collections.