mongodb aggregation pipeline merging two collections both with $match criteria required - mongodb

Question

Hi I have two collections (product and order) and am trying to create an aggregation that will result in all the products being listed for a particular supplier with the qty from a particular order being appended to the data if the order contains that particular product in it.
Product Collection
{
_id: objectId,
supplierId: objectId1, // supplier 1
sku: "prod1"
},
{
_id: objectId,
supplierId: objectId1, // supplier 1
sku: "prod2"
},
{
_id: objectId,
supplierId: objectId1, // supplier 1
sku: "prod3"
}
{
_id: objectId,
supplierId: objectId2, // supplier 2
sku: "prod4"
},
{
_id: objectId,
supplierId: objectId2, // supplier 2
sku: "prod5"
}
Order Collection
{
_id: objectId5 // order 1
product: [{
_id: objectId,
supplierId: objectId1,
sku: prod1,
qty: 5
},
{
_id: objectId,
supplierId: objectId1,
sku: prod2,
qty: 1
}]
},
{
_id: objectId6 // order 2
product: [{
_id: objectId,
supplierId: objectId1,
sku: prod1,
qty: 100
}, ...
]
}
result should be
{
_id: objectId,
supplierId: objectId1, // supplier 1
sku: "prod1",
qty: 5
},
{
_id: objectId,
supplierId: objectId1, // supplier 1
sku: "prod2",
qty: 1
},
{
_id: objectId,
supplierId: objectId1, // supplier 1
sku: "prod3",
qty: 0 || null
}
I've not got very far with my attempt but have noted down some pseudocode as to where my head is at with this problem. I'm no doubt over simplyfing what needs to be done here but any help would be greatly appreciated.
product.aggregate([{$match: {supplierId: objectId1}},
{$lookup: {join order collection but only where orderId: objectId5}},
{$projection: {append qty where there is one append to each product that matches. prodcuts that don't match append 0 or null}}
])

Answer 1

You can try below aggregation with $lookup pipeline in 3.6 version.
$lookup pipeline
db.products.aggregate([
{"$match":{"supplierId":objectId1}},
{"$lookup":{
"from":"orders",
"let":{"supplierId":"$supplierId","sku": "$sku"},
"pipeline":[
{"$match":{"_id":objectId5, "$expr":{"$and":[{"$in":["$$supplierId","$product.supplierId"]},{"$in":["$$sku","$product.sku"]}]}}},
{"$unwind":"$product"},
{"$match":{"$expr":{"$and":[{"$eq":["$$supplierId","$product.supplierId"]},{"$eq":["$$sku","$product.sku"]}]}}}
],
"as":"mproduct"
}},
{"$replaceRoot":{"newRoot":{"$mergeObjects":["$$ROOT",{"$cond":[{"$ne":["$mproduct",[]]},{"qty":{"$arrayElemAt":["$mproduct.product.qty",0]}},{"qty":0}]}]}}},
{"$project":{"mproduct":0}}
])