Aggregate data and populate in one request - mongodb

Question

I am a bit puzzled by populate in MongoDB.
I've got a Schema:
import { Schema, Document, model } from "mongoose";
export interface ProductGroupType {
id: Schema.Types.ObjectId,
title: String,
name: String,
description: String,
}
const ProductGroupSchema: Schema<Document<ProductGroupType>> = new Schema({
title: { type: String, trim: true },
name: { type: String, trim: true },
description: { type: String, trim: true },
}, { collection: "productGroups", timestamps: true });
export const ProductGroupModel = model('ProductGroup', ProductGroupSchema);
and products
import { Schema, Document, model } from "mongoose";
import { plugin as autocomplete } from 'mongoose-auto-increment';
const ProductSchema: Schema<Document<IProduct>> = new Schema({
article: Number,
name: String,
category: { type: Schema.Types.ObjectId, ref: 'ProductCategory' },
group: { type: Schema.Types.ObjectId, ref: 'ProductGroup' },
price: { type: Number, default: 0 },
discount: { type: Number, default: 0 },
stock: {
available: { type: Number, default: 0 },
reserved: { type: Number, default: 0 },
},
images: [Object],
description: String,
productDetails: Object,
}, { collection: "products", timestamps: true });
ProductSchema.plugin(autocomplete, {
model: 'Product',
field: 'article',
startAt: 10000,
});
export const ProductModel = model('Product', ProductSchema);
I need to make a request and group on the MongoDB side data by the field 'group'.
I can make this like this:
await ProductModel.aggregate([
{ $match: { category: Types.ObjectId(queryCategory.id) } },
{
$group: {
_id: '$group',
products: {
$push: {
id: '$_id',
name: '$name',
article: '$article',
price: '$price',
discount: '$discount',
description: '$description',
group: '$groupName',
}
},
count: { $sum: 1 },
}
},
]);
but the output here is:
[
{ _id: 61969583ad32e113f87d0e99, products: [ [Object] ], count: 1 },
{
_id: 61993fff452631090bfff750,
products: [ [Object], [Object] ],
count: 2
}
]
almost what I need but I've been playing around with population and I cannot make it work with Aggregation framework.
I already tried to use the 'lookup' operator but it returns an empty array and doesn't want to work.
That's how I wanted to make it work:
const products: Array<IProduct> = await ProductModel.aggregate([
{ $match: { category: Types.ObjectId(queryCategory.id) } },
{
$group: {
_id: '$group',
products: {
$push: {
id: '$_id',
name: '$name',
article: '$article',
price: '$price',
discount: '$discount',
description: '$description',
group: '$groupName',
}
},
count: { $sum: 1 },
}
},
{
$lookup: {
"from": "productGroups",
"localField": "group",
"foreignField": "_id",
"as": "groupName"
},
},
]);
Is it possible to get the same result as I've got now but populate in the same query group field?
So far the only way I've managed to populate it like this as the second request:
await ProductGroupModel.populate( products.map( (product: any) => {
return {
_id: new ProductGroupModel(product),
products: product.products,
count: product.count,
}
} ), { "path": "_id" } )

Answer 1

In a MongoDB aggregation pipeline, the $group stage passes along only those field explicitly declared in the stage.
In the same pipeline you show, the documents passed along by the $group stage would contain the fields:
_id
products
count
When the exector arrives a the $lookup stage, none of the documents contain a field named group.
However, the value previously contained in the group field still exists, in the _id field.
In the $lookup stage, use
"localField": "_id",
to find documents based on that value.