.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.

[vkarpov15]

So in 3.x there's no way to populate from multiple models - one populate() call <=> one model. @chetverikov added the ability to populate from multiple models into 4.x so this may be more feasible now. I'll re-open this issue and see how feasible it is for a future release.

[diegodfsd]

Hi @vkarpov15

I'm with the same problem but i saw that you close the issue this means that now should be possible populate a child of a specialisation?

[vkarpov15]

Yep see PR #2773 courtesy of @chetverikov

[diegodfsd]

Well,

I'm using Mongoose 4.0.1 and I can't populate a collection in one specialisation.

'use strict';

const Mongoose = require('mongoose'),
      Util = require('util');

function BaseSchema() {
  Mongoose.Schema.apply(this, arguments);

  this.add({
    name     : { type: String, required: true },
    date     : { type: Date, required: true },
    period   : { start : { type: String, required: true },
                 end   : { type: String, required: true } 
               }
  });
}

Util.inherits(BaseSchema, Mongoose.Schema);

let Event = new BaseSchema({});


module.exports = {
  schema: BaseSchema,
  model: Mongoose.model('Event', Event)
};

'use strict';

const Mongoose = require('mongoose'),
      Event = require('./event');

let Talk = new Event.schema({ 
  pin            : { type: String, required: true, index: { unique: true } },
  totalAttendees : { type: Number },
  speakers       : [{ type: Mongoose.Schema.Types.ObjectId, ref: 'Speaker' }],
  surveys        : [{ type: Mongoose.Schema.Types.ObjectId, ref: 'Survey' }],
  questions      : [{ type: Mongoose.Schema.Types.ObjectId, ref: 'Question' }]
});

module.exports = Event.model.discriminator('Talk', Talk);

Here is how I'm trying using populate unsuccessfully.

const  Event = require('./../models/event');

Event.find({}).populate('surveys').exec(function(err, events) {
  if (err) {
    console.dir(err);
  }
  console.dir(results[0].surveys); // this always returns undefined same when there is some item in the collection
});

This is an example of my data collection events

{ "_id" : ObjectId("551b71e01e4e40bc2b168884"), "name" : "Meetup rails", "date" : ISODate("2015-04-01T00:00:00Z"), "pin" : "0004", "__t" : "Talk", "period" : { "start" : "11:00", "end" : "12:00" }, "questions" : [ ], "surveys" : [ ObjectId("551b71ef1e4e40bc2b168885") ], "speakers" : [ ], "__v" : 1 }

This is the output with query result

{ _id: 551b71e01e4e40bc2b168884,
  name: 'Meetup rails',
  date: Tue Mar 31 2015 21:00:00 GMT-0300 (BRT),
  pin: '0004',
  __t: 'Talk',
  questions: [],
  surveys: [],
  speakers: [],
  __v: 1,
  period: { start: '11:00', end: '12:00' } }

I can't see where is my mistake or if the fix did not work.

[chetverikov]

@vkarpov15 @diegodfsd
Oh +( Forgive me please.

Indeed, I made a mistake and wrote two lines incorrectly +( But I already corrected this problem!
#2825

[diegodfsd]

Do you will release a new version with this fix when?

[ericsaboia]

cheers @chetverikov!

[diegodfsd]

No problem @chetverikov

[aforty]

Edit: opened separate issue instead.