How to populate a document inside an array inside another document? - node.js

Question

Sorry if title looks complicated... I couldn't think of a better way to describing it.
My real case situation matches the following Schemes:
Collection1:
const mongoose = require('mongoose');
const itemSchema = mongoose.Schema({
_id: mongoose.Schema.Types.ObjectId,
name: { type: String, required: [true, 'Name is required.'] },
quantity: { type: Number, required: [true, 'Quantity is required.'] },
collection2: { type: mongoose.Schema.Types.ObjectId, ref: 'Collection2' }
}, { _id : false });
const collection1Schema = mongoose.Schema({
_id: mongoose.Schema.Types.ObjectId,
name: { type: String, required: [true, 'Name is required.'] },
imagePath: { type: String, required: [true, 'Image is required.'] },
items: [itemSchema]
});
module.exports = mongoose.model('Collection1', collection1Schema);
Note: itemsSchema is inside the collection1 file (and having no declared _id's) because they only exist for the Collection1 model (considering "quantity" and other fields I removed for simplification). This itemsScheme is not needed elsewhere as another collection.
Collection2:
const mongoose = require('mongoose');
const collection2Schema = mongoose.Schema({
_id: mongoose.Schema.Types.ObjectId,
name: { type: String, required: [true, 'Name is required.'], unique: true }
});
module.exports = mongoose.model('Collection2', collection2Schema );
Note: Other properties (such as 'imagePath') were removed for simplification.
Now, this is the query I am trying to run:
Collection1.find()
.populate({
path: 'items',
populate: {
path: 'collection2', model: 'Collection2'
}
})
.then(...)
.catch(...);
And this is the error message I am getting when I run it:
Error fetching collection1: Cast to ObjectId failed for value "{
name: 'an item name',
quantity: 750
}" at path "_id" for model "Collection1"
The exact same error happens if I just run:
Collection1.find()
.populate('items')
.then(...)
.catch(...);
Maybe I cannot run .populate('items') because it has no declared model. If this is the case, how can I populate collection2 while querying collection1? Again, I cannot consider storing items in a separated collection.
But if I run:
Collection1.find()
.populate('collection2')
.then(...)
.catch(...);
I get the items, no errors, but it doesn't populate collection2. Well, it makes sense for the items because they're just an array of a block of properties inside collection1. But what about populating collection2?
Collection2 already has a few documents added, all with their _ids and other fields well filled. In the controller, I set _id: new mongoose.Types.ObjectId(), while creating a new document for both cases, Collection1 and Collection2.
In the front-end, I create a new document for Collection1, I add items, each item with a document from Collection2, and I save everything with no errors. I also confirmed everything is been properly saved (collection1 has list of items and each item an _id reference to collection2). The only problem is populating collection2 inside this array.
I have already tried restructuring everything with _ids (including itemScheme) and dropping all collections to test it again but no success.
I have been stuck with this problem for about three days now.
Is there any special property I should be setting for populate to make it work for this specific structure?
Thanks in advance...

Answer 1

populate('items')
This will not work as item is not a model.
What you want is following:
Collection1.find()
.populate('items.collection2')
.then(...)
.catch(...);
This will populate collection2 in all the array elements