Mongoose Populate not working in array of sub-document with "path" option

Question

Here's my Mongoose schema:

const productSchema = mongoose.Schema({
  writer: {
        type: Schema.Types.ObjectId,
        ref: 'User'
  },
  schedule: [{
            dateTime: {
                type: String,
                unique: 1            
            },
            candidates: [{
                type: Schema.Types.ObjectId,
                ref: 'User'
            }],
            attorn: {
                type: Schema.Types.ObjectId,
                ref: 'User'
            }
        }]
   ...

This is where I get my items along with the candidates:

    Product.find({ '_id': { $in: productIds } })
    .populate('writer')
    .populate({ 
        path: 'schedule',
        populate: {
          path: 'candidates',
          model: 'User'
        } 
     })
    .exec((err, product) => {
        if (err) return res.status(400).send(err)
        return res.status(200).send(product)
    })

1. Result of this code is a product with only populated 'writer' and empty array for 'candidates'
2. If I remove the line for populating 'candidates', it returns array of _id for 'candidates'
3. How do I populate the 'candidates' array?

Answer 1

can you try this

Product.find({ _id: { $in: productIds } })
  .populate("writer")
  .populate({
    path: "schedule.candidates",
  })
  .populate({
    path: "schedule.attorn",
  })
  .exec((err, product) => {
    if (err) return res.status(400).send(err);
    return res.status(200).send(product);
  });