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Summary: Learn how to efficiently join two collections in MongoDB, specifically working with the `Order` and `OrderAssigned` entities, using the Aggregation Framework and Spring MongoDB.
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How to Join Two Collections in MongoDB for Order and OrderAssigned Entities?

In the world of NoSQL databases, MongoDB is a robust and highly scalable choice. However, SQL users coming to MongoDB often miss the elaborate JOIN operations they used with relational databases. While MongoDB does not provide JOINs in the traditional SQL sense, it does offer a powerful Aggregation Framework to achieve similar functionality. This post will guide you through joining two collections, Order and OrderAssigned, using MongoDB's Aggregation Framework and Spring MongoDB.

Overview of Entities

Let's briefly define what Order and OrderAssigned might look like:

Order Document

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OrderAssigned Document

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In a relational database, you might use a JOIN to fetch Order details along with their assignments. In MongoDB, you can use the Aggregation Framework to achieve this.

Using MongoDB Aggregation Framework

The MongoDB Aggregation Framework allows you to process data records and perform transformations or aggregations. Here’s a step-by-step process to join the Order and OrderAssigned collections.

Aggregation Pipeline Example

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Explanation

$lookup: This stage performs the join operation. It specifies the from collection (orderAssigned), the fields to use for the join (localField and foreignField), and how to name the resulting array (as).

$unwind: This stage deconstructs the assignments array from the $lookup operation back into individual documents.

$project: This stage shapes the output documents by including desired fields and renaming any fields as needed.

Using Spring MongoDB

Spring Data MongoDB provides a flexible and convenient way to work with MongoDB in Java applications. You can use the Aggregation class to build the pipeline programmatically.

Aggregation Example in Spring

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Explanation

Aggregation.lookup(): Mirrors the $lookup stage of the aggregation pipeline.

Aggregation.unwind(): Mirrors the $unwind stage to flatten the array.

Aggregation.project(): Specifies which fields to include in the final result and allows you to rename fields.

mongoTemplate.aggregate(): Executes the aggregation pipeline against the orders collection, mapping the results into the JoinedOrder class.

Conclusion

Joining two collections in MongoDB requires a shift in mindset from traditional SQL JOINs to leveraging the power of MongoDB's Aggregation Framework. This post demonstrated how to join Order and OrderAssigned entities both in raw MongoDB and in a Spring Data MongoDB context. By utilizing the lookup, unwind, and project stages, you can effectively combine documents from multiple collections to gain the insights you need.

Understanding and mastering these tools will empower you to handle more complex MongoDB queries and operations efficiently. Happy coding!