How can I populated nested documents with their Virtuals with Mongoose?

Question

I have following schema:

// foo.js
...

const fooSchema = new Schema(
  {
    name: {
      type: String
    }
  },
  {
    toJSON: {
      virtuals: true
    },
    toObject: {
      virtual: true
    }
  }
);

// sample virtual
fooSchema.virtual('sampleVirtual').get(function () {
  return 'I am a virtual'
});

export default mongoose.model( 'foo', fooSchema );

and another schema where I have a ref on the above schema:

// hello.js
...

const helloWorldSchema = new Schema(
  {
    name: {
      type: String,
      required: true
    },
    myFoo: {
      type: mongoose.Types.ObjectId,
      ref: 'foo'
    }
  },
  {
    toJSON: { 
      virtuals: true 
    },
    toObject: {
        virtual: true
    }
  }
);

export default mongoose.model( 'HelloWorld', helloWorldSchema );

I am now using Mongoose' findOne() method to query for a document of the helloWorldSchema collection and want to populate the myFoo document with it's virtual sampleVirtual:

// get.js
const retVal = await helloWorldModel
    .findOne({_id: <some-id> })
    .populate({ path: 'myFoo', select: '_id name sampleVirtual' })
    .exec();

console.log(retVal.myFoo.sampleVirtual) //undefined

Am I doing something wrong?

Best Valentin

Answer 1

Your code has some typos. The toJSON and toObject are schema options, and the options is the second parameter of the mongoose.Schema class constructor. The mongoose.Schema class constructor signature is:

constructor(definition?: SchemaDefinition<LeanDocument<SchemaDefinitionType>>, options?: SchemaOptions);

E.g. ("mongoose": "^7.3.1")

import mongoose from 'mongoose';
import { config } from '../../config';

mongoose.set('debug', true);

const fooSchema = new mongoose.Schema(
    {
        name: String,
    },
    {
        toJSON: { virtuals: true },
        toObject: { virtuals: true },
    },
);
fooSchema.virtual('sampleVirtual').get(function () {
    return 'I am a virtual';
});
const Foo = mongoose.model('foo', fooSchema);

const helloWorldSchema = new mongoose.Schema(
    {
        name: String,
        myFoo: { type: mongoose.Schema.Types.ObjectId, ref: 'foo' },
    },
    {
        toJSON: { virtuals: true },
        toObject: { virtuals: true },
    },
);
const HelloWorld = mongoose.model('helloWorld', helloWorldSchema);

(async function main() {
    try {
        await mongoose.connect(config.MONGODB_URI);
        // seed
        const [foo1] = await Foo.create([{ name: 'a' }, { name: 'b' }]);
        const [h1] = await HelloWorld.create([
            { name: 'x', myFoo: foo1._id },
            { name: 'y', myFoo: foo1._id },
        ]);
        const retVal = await HelloWorld.findOne({ _id: h1._id })
            .populate({ path: 'myFoo', select: '_id name sampleVirtual' })
            .exec();
        console.log(retVal);
    } catch (error) {
        console.error(error);
    } finally {
        await mongoose.connection.close();
    }
})();

Debug logs:

Mongoose: foos.insertOne({ name: 'a', _id: ObjectId("649a7bdfbedaa852fa187538"), __v: 0 }, {})
Mongoose: foos.insertOne({ name: 'b', _id: ObjectId("649a7bdfbedaa852fa187539"), __v: 0 }, {})
Mongoose: helloworlds.insertOne({ name: 'x', myFoo: ObjectId("649a7bdfbedaa852fa187538"), _id: ObjectId("649a7bdfbedaa852fa18753c"), __v: 0}, {})
Mongoose: helloworlds.insertOne({ name: 'y', myFoo: ObjectId("649a7bdfbedaa852fa187538"), _id: ObjectId("649a7bdfbedaa852fa18753d"), __v: 0}, {})
Mongoose: helloworlds.findOne({ _id: ObjectId("649a7bdfbedaa852fa18753c") }, {})
Mongoose: foos.find({ _id: { '$in': [ ObjectId("649a7bdfbedaa852fa187538") ], [Symbol(mongoose#trustedSymbol)]: true }}, { skip: undefined, limit: undefined, perDocumentLimit: undefined, projection: { _id: 1, name: 1, sampleVirtual: 1 }})
{
  _id: new ObjectId("649a7bdfbedaa852fa18753c"),
  name: 'x',
  myFoo: {
    _id: new ObjectId("649a7bdfbedaa852fa187538"),
    name: 'a',
    sampleVirtual: 'I am a virtual',
    id: '649a7bdfbedaa852fa187538'
  },
  __v: 0,
  id: '649a7bdfbedaa852fa18753c'
}

The virtual field sampleVirtual is there.