Can't populate Mongoose schema on express route

Question

So I want to populate two things and return them in the response, I want to return an array of images and the author. However the response returns nothing, if I exclude the .populate() methods then it returns fine with the ObjectId. Not sure what the cause is. How can I debug this, am I doing something wrong?

// Product Controller:
router.get('/:id', (req, res) => {
  Product.findById(req.params.id)
    .populate('author')
    .populate(['images'])
    .then(product => res.json({ success: true, product: product }))
    .catch(err => res.status(404).json({ success: false, err: err }));
});

// Product Model:
const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const ProductSchema = new Schema({
  name: {
    type: String,
    required: true,
  },
  images: [{
    type: Schema.Types.ObjectId,
    ref: 'Image',
  }],
  author: {
    type: Schema.Types.ObjectId,
    ref: 'User',
  }
});

module.exports = Product = mongoose.model('product', ProductSchema);

// Product document:
{
    "_id" : ObjectId("620ab784cde3e34263a9a3d9"),
    "images" : [ ObjectId("620ab784cde3e34263a9a3d6") ],
    "name" : "testing",
    "author" : ObjectId("5db8f65bb027c31ede3b64ae"),
    "__v" : 0
}

Answer 1

You can populate an array of ObjectIds using the same syntax as a single ObjectId. In other words, removing the brakets around the .populate(['images']) line should do the trick. Like so:

//From:
Product.findById(req.params.id)
    .populate('author')
    .populate(['images'])

//To:
Product.findById(req.params.id)
    .populate('author')
    .populate('images')

Also, in your model file, your model references have a cap in them but the populate doesn't. Change your model file to match this, if I recall correctly, Mongoose converts model names to lower case so the ref here may be looking for a model that doesn't exist. Like so:

//From:
const ProductSchema = new Schema({
  name: {
    type: String,
    required: true,
  },
  images: [{
    type: Schema.Types.ObjectId,
    ref: 'Image',
  }],
  author: {
    type: Schema.Types.ObjectId,
    ref: 'User',
  }
});

//To:
const ProductSchema = new Schema({
  name: {
    type: String,
    required: true,
  },
  images: [{
    type: Schema.Types.ObjectId,
    ref: 'image',
  }],
  author: {
    type: Schema.Types.ObjectId,
    ref: 'user',
  }
});