MongoDB Aggregation Framework - $lookup stage

💖 $lookup stage: 

In the MongoDB aggregation framework, the $lookup stage is used to combine multiple collections based on the common fields of the document and include the related data in the output document.

💢 Syntax:

{ $lookup: { from: <foreignCollection>, localField: <localField>, foreignField: <foreignField>, as: <outputArrayField> } }

from: The name of the foreign collection you want to join with.

localField: The field from the input documents that will be used to match with the foreignField in the foreign collection.

foreignFiel: The field from the foreign collection that will be used to match with the localField in the input documents.

as: The name of the new array field that will hold the joined documents from the foreign collection.

💧 Combine or join multiple collections:

db.orders.aggregate([ { $lookup: { from: "products", localField: "productId", foreignField: "_id", as: "productDetails" } }, { $lookup: { from: "customers", localField: "customerId", foreignField: "_id", as: "customerDetails" } } ])