Mongoose fails to populate virtual when foreignField is an Array

Question

I'm trying to create a virtual populator in mongoose, but I'm stumped on this aspect, and I don't know if this is a limitation, a bug, or simply I'm doing something wrong.

The idea is that a user can be member of several groups, and then I can populate a virtual in the group schema by querying users that have its id on the groups array.

I created an example (based on the original mongoose documentation).

Luca Turilli has been into many bands, sometimes at the same time, so the original model (in which band was a single field) doesn't cut for italian power metal.

    const mongoose = require('mongoose');
    const Schema = mongoose.Schema;
    mongoose.connect('mongodb://localhost/test');
    var db = mongoose.connection;
    db.on('error', console.error.bind(console, 'connection error:'));
    db.once('open', function() {
      console.log('connected to db');
    });

    var artistSchema = new Schema ({
        name: String,
        bands: [{type: String, ref: 'band'}]
    });
    
    var bandSchema = new Schema ({
        name: String
    },{
        toJson:{virtuals:true},
        toObject: {virtuals:true}
    });
    bandSchema.virtual('lineup', {
        ref: 'Artist',
        localField: 'name',
        foreignField: 'bands'
    });
    
    var Artist = mongoose.model('Artist', artistSchema);
    var Band = mongoose.model('Band', bandSchema);
    /* Comment this if you already populated the database */
    Band.create({name:'Dreamquest'});
    Band.create({name:'Luca Turilli'});
    Band.create({name:'Rhapsody of Fire'});
    
    Artist.create({name:'Luca Turilli', bands:['Dreamquest','Luca Turilli','Rhapsody of Fire']});
    Artist.create({name:'Olaf Hayer', bands:['Luca Turilli']});
    Artist.create({name:'Sascha Paeth', bands:['Luca Turilli','Dreamquest']});
    Artist.create({name:'Robert Hunecke-Rizzo', bands:['Luca Turilli', 'Dreamquest']});
    Artist.create({name:'Dominique Leurquin', bands:['Dreamquest', 'Rhapsody of Fire']});
    Artist.create({name:'Patrice Guers', bands:['Rhapsody of Fire']});
    Artist.create({name:'Alex Landenburg', bands:['Rhapsody of Fire']});
    /*stop commenting here*/
    
    Band.find(function(err, bands) {
        if (err) return console.error(err);
        console.log(bands);
    });

The expected output would be something along the lines of:

    [ { _id: 5b8fd9eef72e14315b52985f,
        name: 'Dreamquest',
        __v: 0,
        lineup: [Artist entries of Luca,Sascha,Robert and Dominique]},
      .... more bands
      ]

Instead, I get this

        [ { _id: 5b8fd9eef72e14315b52985f,
        name: 'Dreamquest',
        __v: 0,
        lineup: null,
        id: '5b8fd9eef72e14315b52985f' },
      { _id: 5b8fd9eef72e14315b529860,
        name: 'Luca Turilli',
        __v: 0,
        lineup: null,
        id: '5b8fd9eef72e14315b529860' },
      { _id: 5b8fd9eef72e14315b529861,
        name: 'Rhapsody of Fire',
        __v: 0,
        lineup: null,
        id: '5b8fd9eef72e14315b529861' } ]

I've tried to search for examples or people in the same situation, but I haven't found much information. I know I probably could get a getter function to do this, but I want to know if it is possible to do it with virtuals or I'm wasting my time.

score 2 · Accepted Answer · answered Sep 05 '18 at 16:16

In the end, the response is simple: virtuals:truedoesn't autopopulate the fields, and virtuals don't populate by themselves. It's confusing because it would seem that populated virtual implies that the virtual populates by itself...

Anyway the correct query is:

Band.find({}).populate('lineup').exec(function(err,bands) {
    if (err) return console.error(err);
    console.log(bands);
}

As with any manual population, you can hook into 'find', 'findone' and 'save' (among others) to set it to autopopulate, if you're planning to access this info regularly.

Mongoose fails to populate virtual when foreignField is an Array

1 Answers1