Getters for populate virtuals

[NikoFoX]

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

What is the current behavior?
Current populate virtual doesn't allow you to assign foreign object's field value as a virtual's returned value.

Currently doing:

const gameSchema = Schema({
      title: String,
});

const matchSchema = Schema({
      game: ObjectId,
});

matchSchema.virtual('gameTitle', {
      ref: 'Game',
      localField: 'game',
      foreignField: '_id',
      justOne: true,
      options: {select: {'title': 1}}
});

and returning populated Match gives:

{
      _id: 'some_match_id',
      gameTitle: {
            _id: 'some_game_id',
            title: 'some_game_title'
      }
}

What is the expected behavior?
Same virtual should return Match object with gameTitle as String with current Game title.

{
      _id: 'some_match_id',
      gameTitle: 'some_game_title'
}

I'm also wondering why the select option results in returning both selected field and _id field and not the field alone.

[vkarpov15]

The reason why _id is there is that _id is included by default, and Mongoose needs _id to match up the Game to its corresponding Match. Any other properties in gameSchema will be excluded.

The gameTitle: 'some_game_title' output is not something we support. Virtual populate always sets gameTitle to a Mongoose document, we don't currently support transforming the document into a string.

We'll keep this open for a future feature suggestion. I'm thinking we can add support for something like this:

matchSchema.virtual('gameTitle', {
      ref: 'Game',
      localField: 'game',
      foreignField: '_id',
      justOne: true,
      options: {select: {'title': 1}},
      set: doc => doc.title
});

Would that work for you @NikoFoX ?

[NikoFoX]

Shouldn't it be get instead of set? Analogically to schema path getters.
That would work for me very much.

[vkarpov15]

get sounds better, thanks for the suggestion, we'll go with that.