Node.js MongoDB Join

In MongoDB, to join data from multiple collections in Node.js, we can use the $lookup aggregation pipeline stage. The $lookup stage allows us to perform a left outer join on two collections and merge the results into a single document. In this tutorial, we'll discuss how to use $lookup in Node.js to join data from multiple collections.

Syntax

The syntax for using $lookup in a MongoDB aggregation pipeline in Node.js is as follows:

db.collection('collection1').aggregate([
   { $lookup:
      {
        from: "collection2",
        localField: "field1",
        foreignField: "field2",
        as: "output"
      }
   }
])

• from: specifies the name of the collection to join.
• localField: specifies the field from the input document to use for the join.
• foreignField: specifies the field from the documents of the "from" collection to use for the join.
• as: specifies the name of the output field that will contain the joined documents.

Example

Let's say we have two collections in MongoDB, "orders" and "customers". "orders" contains a "customerId" field that references the "_id" field on the "customers" collection. We want to join these two collections in Node.js to retrieve the customer details for each order. Here's how we can do it:

db.collection('orders').aggregate([
   { $lookup:
      {
        from: "customers",
        localField: "customerId",
        foreignField: "_id",
        as: "customer"
      }
   }
])

Now, for each order document, the "customer" field will contain an array of customer documents that match the "customerId" field.

Output

When we run the above example, the output will be an array of documents that contains the joined data from the "orders" and "customers" collections.

Explanation

In the example above, we used the $lookup stage in a MongoDB aggregation pipeline in Node.js to join data from the "orders" and "customers" collections. We specified the "from" field to be "customers", the "localField" to be "customerId" from the "orders" collection, and the "foreignField" to be "_id" from the "customers" collection. We also specified the "as" field as "customer" to indicate that we want to merge the joined documents into a "customer" field in the output.

Use

Using $lookup in a MongoDB aggregation pipeline in Node.js is useful when you need to fetch data from multiple collections and merge them into a single document. It provides a way to perform a left outer join on two collections, based on a common field or set of fields.

Important Points

• $lookup is an expensive operation, and you should use it only when necessary.
• The $lookup stage returns an empty array if no matching documents are found in the joined collection.
• Make sure to index the fields that are used for the join to speed up the query.

Summary

In this tutorial, we discussed how to use $lookup in a MongoDB aggregation pipeline in Node.js to join data from multiple collections. We covered the syntax, example, output, explanation, use, and important points of using $lookup in Node.js. With this knowledge, you can now join data from multiple collections in MongoDB using Node.js and perform a left outer join on two collections based on a common field or set of fields.