Database
db={"catalog":[{"_id":ObjectId("5ec7856eb9eb171b72f721af"),"model":"HX711","type":"DIGITAL"}],"stock":[{"_id":ObjectId("5ec78573b9eb171b72f721ba"),"serialNumber":"7af646bb-a5a8-4b86-b56b-07c12a625265","bareCode":"72193.67751691974","productId":"5ec7856eb9eb171b72f721af"},{"_id":ObjectId("5ec84f1043732e8f9a64028d"),"serialNumber":"7af646bb-a5a8-4b86-b56b-07c12a625265","bareCode":"72193.67751691974","productId":"5ec84f1043732e8f9a64028d"}]}
Query
db.stock.aggregate([{"$lookup":{"from":"catalog","let":{"productId":{"$toObjectId":"$productId"}},"pipeline":[{"$match":{"$expr":{"$eq":["$_id","$$productId"]}}}],"as":"product"}},{"$unwind":{"path":"$product","preserveNullAndEmptyArrays":true}}])