db={Orders:[{_id:"ord1",items:[{catalog_id:"xyz1",qty:5},{catalog_id:"xyz2",qty:3}]}],Catalogs:[{_id:"xyz1",name:"my catalog name",SKU:"XxYxZx1"},{_id:"xyz2",name:"my other catalog name",SKU:"XxYxZx2"}]}

db.Orders.aggregate([{"$unwind":"$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"}]}}}])