Optimize nested populate

[skotchio]

In the following PostSchema we have nested structure that has _author field in post itself and comments:

var PostSchema = mongoose.Schema({
    _author: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'User',
    },
    comments: {
        type: [{
            _author: {
                type: mongoose.Schema.Types.ObjectId,
                ref: 'User'
            },

            text: {
                type: String
            }
        }]
    }
});

 var PostModel = mongoose.model('Post', PostSchema);

When we fetch post and populate _author fields mongoose makes two queries:

PostModel.findById(ObjectId(...)).populate('_author comments._author').exec(callback);

Mongoose: users.find({ _id: { '$in': [ ObjectId("1") ] } }) { fields: undefined }  
Mongoose: users.find({ _id: { '$in': [ ObjectId("2") ] } }) { fields: undefined }  

It would be great performance increment if mongoose will make only one query to populate _author field:

Mongoose: users.find({ _id: { '$in': [ ObjectId("1"),  ObjectId("2")] } }) { fields: undefined }  

[krishna-koushik]

any idea when this might get done? really hoping for this performance gain.

[vkarpov15]

Not really a priority atm but PRs are welcome.

[vkarpov15]

We did a bunch of refactoring to support this in the 5.7 branch, but it's proven to be quite tricky. The amount of refactoring we did is about as much as I'm comfortable doing for a minor release, especially since populate() is pretty confusing and this has already delayed 5.7 by a week.