NodeJS
Optimize MongoDB Joins with $lookup for Faster Queries
Introduction
A system consisting of MySQL and PostgreSQL implements joins for efficient access to multiple table data. Since MongoDB operates as a NoSQL database it does not adopt traditional relational join standards. The `$lookup` aggregation stage functions as a left outer join mechanism for collections in MongoDB. The $lookup aggregation stage represents a significant functionality that proves advantageous when working with MongoDB databases that utilize normalization.
Understanding `$lookup`
Using the `$lookup` stage enables MongoDB users to conduct collections joins which fetches relationship data by avoiding multiple data request efforts. The feature becomes useful for situations that require cross-collection document references through shared fields.
Syntax of `$lookup`
{ $lookup: { from: "<foreign_collection>", localField: "<local_field>", foreignField: "<foreign_field>", as: "<output_field>" }}
The name of the collection to join appears here.
The match occurs through the `localField` parameter containing the current collection object field.
The matching field within the foreign collection exists under the name `localField`.
Therefore the "as" parameter defines the name of output array field for storing matched documents.
The following query shows the order of joining orders with users Collections
Consider a collection of orders which contains user IDs that need user detail retrieval from the users collection through `$lookup`.
Orders Collection
{ "_id": ObjectId("654321"), "orderId": "ORD12345", "userId": ObjectId("507f1f77bcf86cd799439011"), "amount": 250}
users Collection
{ "_id": ObjectId("507f1f77bcf86cd799439011"), "name": "John Doe", "email": "johndoe@example.com"}
The `$lookup` stage allows you to retrieve user information by linking orders collection data with user collection content.
db.orders.aggregate([ { $lookup: { from: "users", localField: "userId", foreignField: "_id", as: "userDetails" } }, { $unwind: "$userDetails" }]);Output
{ "_id": "654321", "orderId": "ORD12345", "userId": "507f1f77bcf86cd799439011", "amount": 250, "userDetails": { "_id": "507f1f77bcf86cd799439011", "name": "John Doe", "email": "johndoe@example.com" }}
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