-----inventory-------- /* 1 */ { "_id" : 1, "sku" : "abc", "description" : "product 1", "instock" : 120 } /* 3 */ { "_id" : 3, "sku" : "ijk", "description" : "product 3", "instock" : 60 } /* 5 */ { "_id" : 5, "sku" : null, "description" : "Incomplete" } /* 6 */ { "_id" : 6 } ----Orders collection---- /* 1 */ { "_id" : 1, "item" : [ { "ID" : "abc" }, { "ID" : "jkl" } ], "price" : 12, "quantity" : 2 } /* 2 */ { "_id" : 2, "item" : [ { "ID" : "jkl" } ], "price" : 20, "quantity" : 1 } /* 3 */ { "_id" : 3 } ----Aggregate function --- db.orders.aggregate([ { $lookup: { from: "inventory", localField: "item.ID", foreignField: "sku", as: "inventory_docs" } } ])