perform lookup on array from another collection in MongoDB - mongodb

Question

I have a collection of Orders. each order has a list of Items, and each Item has catalog_id, which is an ObjectId pointing to the Catalogs collection.
I need an aggregate query that will retrieve certain orders - each order with its Items in extended fashion including the Catalog name and SKU. i.e:
Original data structure:
Orders: [{
_id : ObjectId('ord1'),
items : [{
catalog_id: ObjectId('xyz1'),
qty: 5
},
{
catalog_id: ObjectId('xyz2'),
qty: 3
}]
Catalogs: [{
_id : ObjectId('xyz1')
name: 'my catalog name',
SKU: 'XxYxZx1'
},{
_id : ObjectId('xyz2')
name: 'my other catalog name',
SKU: 'XxYxZx2'
}
]
ideal outcome would be:
Orders: [{
_id : ObjectId('ord1'),
items : [{
catalog_id: ObjectId('xyz1'),
catalog_name: 'my catalog name',
catalog_SKU: 'XxYxZx1' ,
qty: 5
},
{
catalog_id: ObjectId('xyz2'),
catalog_name: 'my other catalog name',
catalog_SKU: 'XxYxZx2' ,
qty: 3
}
]
What I did so far was:
db.orders.aggregate(
[
{
$match: {merchant_order_id: 'NIM333'}
},
{
$lookup: {
from: "catalogs",
//localField: 'items.catalog_id',
//foreignField: '_id',
let: { 'catalogId' : 'items.catalog_id' },
pipeline: [
{
$match : {$expr:{$eq:["$catalogs._id", "$$catalogId"]}}
},
{
$project: {"name": 1, "merchant_SKU": 1 }
}
],
as: "items_ex"
},
},
])
but items_ex comes out empty for some reason i cannot understand.

Answer 1

You need to first $unwind the items and reconstruct the array back using $group to match the exact position of qty with the catalogs_id inside the items array
db.orders.aggregate([
{ "$match": { "merchant_order_id": "NIM333" }},
{ "$unwind": "$items" },
{ "$lookup": {
"from": "catalogs",
"let": { "catalogId": "$items.catalog_id", "qty": "$items.qty" },
"pipeline": [
{ "$match": { "$expr": { "$eq": ["$_id", "$$catalogId"] } }},
{ "$project": { "name": 1, "merchant_SKU": 1, "qty": "$$qty" }}
],
"as": "items"
}},
{ "$unwind": "$items" },
{ "$group": {
"_id": "$_id",
"items": { "$push": "$items" },
"data": { "$first": "$$ROOT" }
}},
{ "$replaceRoot": {
"newRoot": {
"$mergeObjects": ["$data", { "items": "$items" }]
}
}}
])
MongoPlayground

Answer 2

You're missing a dollar sign when you define your pipeline variable. There should be:
let: { 'catalogId' : '$items.catalog_id' },
and also this expression returns an array to you need $in instead of $eq:
{
$lookup: {
from: "catalogs",
let: { 'catalogId' : 'items.catalog_id' },
pipeline: [
{
$match : {$expr:{$in:["$_id", "$$catalogId"]}}
},
{
$project: {"name": 1, "merchant_SKU": 1 }
}
],
as: "items_ex"
}
}
Mongo Playground