A bug in aggregation's Wizard $lookup as it is not showing the accurate values for local & foreign fields. See the attached image db.orders.insertMany([ { "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 }, { "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 }, { "_id" : 3 } ]) db.inventory.insertMany([ { "_id" : 1, "sku" : "almonds", "description": "product 1", "instock" : 120 }, { "_id" : 2, "sku" : "bread", "description": "product 2", "instock" : 80 }, { "_id" : 3, "sku" : "cashews", "description": "product 3", "instock" : 60 }, { "_id" : 4, "sku" : "pecans", "description": "product 4", "instock" : 70 }, { "_id" : 5, "sku": null, "description": "Incomplete" }, { "_id" : 6 } ]) db.orders.aggregate([ { $lookup: { from: "inventory", localField: "item", foreignField: "sku", as: "inventory_docs" } } ])