Can not access document from virtual populated array.

[quanhv94]

Do you want to request a feature or report a bug?
Bug
What is the current behavior?
Can not access document from virtual populated array.
If the current behavior is a bug, please provide the steps to reproduce.

const FooSchema = new Schema({
  name: String,
  children: [{
    barId: { type: Schema.Types.ObjectId, ref: 'Bar' },
    quantity: Number,
  }],
}, {
  toJSON: { virtuals: true },
  toObject: { virtuals: true },
});
FooSchema.virtual('children.bar', {
  ref: 'Bar',
  localField: 'children.barId',
  foreignField: '_id',
  justOne: true,
});

const BarSchema = new Schema({
  name: String,
});

const Foo = mongoose.model('Foo', FooSchema);
const Bar = mongoose.model('Bar', BarSchema);
(async () => {
  const bar = await Bar.create({ name: 'bar' });
  const foo = await Foo.create({ name: 'foo', children: [{ barId: bar._id, quantity: 1 }] });
  const foo2 = await Foo.findById(foo._id).populate('children.bar');
  console.log(foo2.children);
  // [
  //   {
  //     _id: 5d8c395c70fda60a1e83e433,
  //     barId: 5d8c395c70fda60a1e83e431,
  //     quantity: 1,
  //     bar: { _id: 5d8c395c70fda60a1e83e431, name: 'bar', __v: 0 }
  //   }
  // ]
  console.log(foo2.children[0].bar); // this get undefined
  // undefined
  console.log(foo2.toJSON().children[0].bar); // this actually work, but just a json object
  // { _id: 5d8c395c70fda60a1e83e431, name: 'bar', __v: 0 }
})();

What is the expected behavior?
Can access document from virtual populated array.
What are the versions of Node.js, Mongoose and MongoDB you are using?
Nodejs: 12.9.1
Mongoose: 5.7.1
MongoDB: 4.0.10

Please correct me if I have a mistake.