How can I populate nested array objects in mongoDB or mongoose? - mongodb

Question

I have an orders collection where each order has the following shape:
{
"_id": "5252875356f64d6d28000001",
"lineItems": [
{ productId: 'prod_007', quantity: 3 },
{ productId: 'prod_003', quantity: 2 }
]
// other fields omitted
}
I also have a products collection, where each product contains a unique productId field.
How can I populate each lineItem.productId with a matching product from the products collection? Thanks! :)
EDIT: orderSchema and productSchema:
const orderSchema = new Schema({
checkoutId: {
type: String,
required: true,
},
customerId: {
type: String,
required: true,
},
lineItems: {
type: [itemSubSchema],
required: true,
},
});
const itemSubSchema = new Schema(
{
productId: {
type: String,
required: true,
},
quantity: {
type: Number,
required: true,
},
},
{ _id: false }
);
const productSchema = new Schema({
productId: {
type: String,
required: true,
},
name: {
type: String,
required: true,
},
imageURL: {
type: String,
required: true,
},
price: {
type: Number,
default: 0,
},
});

Answer 1

I don't know the exact output you want but I think this is what you are looking for:
The trick here is to use $lookup in an aggregation stage.
First $unwind to deconstruct the array and can merge each id with the other collection.
Then the $lookup itself. This is like a join in SQL. It merges the desired objects with same ids.
Then recreate the population using $mergeObjects to get properties from both collections.
And last re-group objects to get the array again.
db.orders.aggregate([
{
"$unwind": "$lineItems"
},
{
"$lookup": {
"from": "products",
"localField": "lineItems.productId",
"foreignField": "_id",
"as": "result"
}
},
{
"$set": {
"lineItems": {
"$mergeObjects": [
"$lineItems",
{
"$first": "$result"
}
]
}
}
},
{
"$group": {
"_id": "$_id",
"lineItems": {
"$push": "$lineItems"
}
}
}
])
Example here
With this query you have the same intial data but "filled" with the values from the other collection.
Edit: You can also avoid one stage, maybe it is clear with the $set stage but this example do the same as it merge the objects in the $group stage while pushing to the array.

Answer 2

You can use the Mongoose populate method either when you query your documents or as middleware. However, Mongoose only allows normal population on the _id field.
const itemSubSchema = new Schema({
product: {
type: mongoose.Schema.Types.ObjectId,
ref: 'productSchema',
}
});
const order = await orderSchema.find().populate('lineItems.$*.product');
// special populate syntax necessary for nested documents
Using middleware you would still need to reconfigure your item schema to save the _id from products. But this method would automatically call populate each time you query items:
itemSubSchema.pre('find', function(){
this.populate('product');
});
You could also declare your item schema within your order schema to reduce one layer of joining data:
const orderSchema = new Schema({
lineItems: [{
type: {
quantity: {type: Number, required: true},
product: {
type: mongoose.Schema.Types.ObjectId,
required: true,
ref: 'productSchema',
}
},
required: true,
}]
});
const orders = orderSchema.find().populate('lineItems');