Why populate returns empty results in mongodb - node.js

Question

I want to populate my property (subCategories) but it's return empty although there are results in my database. Did I have miss something? I followed the populate method in mongoose:
const Document = mongoose.model('Document', new mongoose.Schema({
name: { type: String },
description: { type: String },
subCategory: { type: mongoose.Schema.Types.ObjectId }
}));
const Category = mongoose.model('Category', new mongoose.Schema({
name: { type: String },
subCategories: [
{
name: { type: String }
}
]
})
);
var cat1 = await new Category({ name: 'cat1', subCategories: [{ name: 'sub-1-cat-1' }, { name: 'sub-1-cat-2' } ]}).save();
var cat2 = await new Category({ name: 'cat2', subCategories: [{ name: 'sub-2-cat-1' }, { name: 'sub-2-cat-2' } ]}).save();
await new Document({ name: 'doc1', description: 'blabla', subCategory: cat2.subCategories[1] }).save();
const results = Document.find({}).populate('subCategory');
// results[0].subCategory is empty?! why?

Answer 1

The subcategory must be a mongoose model in order to be populated, currently the subCategory that you're trying to populate is just an array item for Category model so I would refactor the code you've posted above to this:
const SubCategory = mongoose.model('SubCategory', new mongoose.Schema({
name: { type: String }
}));
const Document = mongoose.model('Document', new mongoose.Schema({
name: { type: String },
description: { type: String },
subCategory: { type: mongoose.Schema.Types.ObjectId, ref: "SubCategory" }
}));
const Category = mongoose.model('Category', new mongoose.Schema({
name: { type: String },
subCategories: [
{ type: mongoose.Schema.Types.ObjectId, ref: "SubCategory" }
]
})
);
var sub1Cat1 = await new SubCategory({ name: 'sub-1-cat-1' }).save();
var sub1Cat2 = await new SubCategory({ name: 'sub-1-cat-2' }).save();
var sub2Cat1 = await new SubCategory({ name: 'sub-2-cat-1' }).save();
var sub2Cat2 = await new SubCategory({ name: 'sub-2-cat-2' }).save();
var cat1 = await new Category({ name: 'cat1', subCategories: [sub1Cat1, sub1Cat2 ] }).save();
var cat2 = await new Category({ name: 'cat2', subCategories: [sub2Cat1, sub2Cat2 ] }).save();
await new Document({ name: 'doc1', description: 'blabla', subCategory: cat2.subCategories[1] }).save();
const results = Document.find({}).populate('subCategory');
// results[0].subCategory is not empty!