Joins and Lookups in MongoDB

Introduction

MongoDB is a NoSQL database that offers flexible data modeling capabilities. Unlike SQL databases, which utilize JOIN operations to combine data from multiple tables, MongoDB uses a different approach due to its document-oriented nature. In this lesson, we will explore how to perform joins and lookups in MongoDB, focusing on the $lookup aggregation stage.

Joins in MongoDB

MongoDB does not support joins in the traditional sense as relational databases do. However, it provides the $lookup operator within the aggregation framework, allowing you to perform left outer joins between documents from two collections.

Lookups

The $lookup stage can be used in an aggregation pipeline to join documents from different collections. Here's the syntax:

{
    $lookup: {
        from: "",
        localField: "",
        foreignField: "",
        as: ""
    }
}

In this syntax:

• from: The name of the collection to join.
• localField: The field from the input documents.
• foreignField: The field from the documents of the "from" collection.
• as: The name of the output array field that will contain the matched documents.

Example

Suppose we have two collections: orders and products. We want to join them based on the productId:

{
    $lookup: {
        from: "products",
        localField: "productId",
        foreignField: "_id",
        as: "productDetails"
    }
}

This will add a new field productDetails to the documents in the orders collection containing the matching product documents.

Best Practices

• Prefer embedding related data when possible to avoid the overhead of joins.
• When using $lookup, ensure indexes are created on the fields used in the join for better performance.
• Limit the number of documents passed to $lookup to reduce memory usage.
• Use $project to shape the output data after a $lookup operation.

FAQ