Populating unset field which type is array of objects results in an array with one empty object

[alexeychikk]

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

What is the current behavior?
Populating unset field which type is array of objects results in an array with one empty object (or object whose fields are all nulls).

If the current behavior is a bug, please provide the steps to reproduce.

https://github.com/alexeychikk/mongoose-populate-array-bug
npm install
npm start

You should notice files: [ { "uploadedBy": null, "id": null } ] in the populated rides. Issue is gone if you comment out await Ride.updateMany({}, { $unset: { files: 1 } });

What is the expected behavior?
Field should be populated with default value or remain empty.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node 10.14
mongodb 3.4.0
mongoose 5.8.1

[AbdelrahmanHafez]

Not sure why this happens, but it's worth noting that this happens only when using virtuals.

In the code below, changing the populate path to
path: 'files.uploaderId' instead of path: 'files.uploadedBy' changes the behavior.

const mongoose = require('mongoose');
const { Schema } = require('mongoose');

const url = 'mongodb://127.0.0.1:27017/test';
mongoose.connect(url, { useNewUrlParser: true });

const companySchema = new Schema({ name: { type: String, required: true } });

const userSchema = new Schema({
  fullName: { type: String, required: true },
  companyId: { type: Schema.Types.ObjectId, ref: 'Company', required: true }
});

const fileSchema = new Schema({
  _id: { type: String, required: true },
  uploaderId: { type: Schema.Types.ObjectId, ref: 'User', required: true }
}, { _id: false, toObject: { virtuals: true }, toJSON: { virtuals: true } });

fileSchema.virtual('uploadedBy', { ref: 'User', localField: 'uploaderId', foreignField: '_id', justOne: true });

const rideSchema = new Schema({
  title: { type: String, required: true },
  files: { type: [fileSchema], default: [] }
}, { toObject: { virtuals: true }, toJSON: { virtuals: true } });

const User = mongoose.model('User', userSchema);
const Company = mongoose.model('Company', companySchema);
const Ride = mongoose.model('Ride', rideSchema);



async function run () {

  await Promise.all([
    Ride.deleteMany({}),
    User.deleteMany({}),
    Company.deleteMany({})
  ]);


  const company = new Company({ name: 'Apple' });
  const user = new User({ fullName: 'John Doe', companyId: company._id });
  const ride = new Ride({
    title: 'Berlin-Moscow',
    files: [{ _id: '123', uploaderId: user._id }]
  });

  await Promise.all([
    user.save(),
    company.save(),
    ride.save()
  ]);


  await Ride.updateMany({}, { $unset: { files: 1 } });

  const foundRide = await Ride.findOne()
    .populate({
      path: 'files.uploadedBy',
      justOne: true,
      populate: {
        path: 'companyId',
        justOne: true
      }
    });

  console.log(foundRide.files);
}

run().catch(console.error);

[alexeychikk]

@AbdelrahmanHafez In your code snippet you still populating files.uploadedBy but you call Ride.findOne() instead of Ride.find().

[alexeychikk]

@AbdelrahmanHafez Also, I can't populate uploaderId because in the original code ride is saved after it is populated, and if user is not found then uploaderId: null and an error occurs while saving because uploaderId is required field.