MongoDB - Aggregations with Java + Spring

#java #mongodb #spring #programming

Coming from PostgreSQL, one of the first things I noticed when I started working with MongoDB was the lack of built-in query methods like those provided by JpaRepository. With MongoRepository, we often only get the basic findAll() method by default.

This post will guide you through creating Aggregations (custom queries) to overcome these limitations. It mirrors the functionality of the Aggregation Pipeline in MongoDB Atlas, but implemented on the Java side using Spring Data MongoDB.

In the aggregation pipeline, you list out a series of instructions in a "stage." For each stage that's defined, MongoDB executes them one after another in order to give a finalized output you're able to use.

Pre-considerations

We'll use a sample dataset provided by MongoDB and map a Java class to some of its fields.

In our class we will use the @Document with our collection name.

In the service layer, we have a method that will call our repository and pass a string that will be the filter for our query.

Custom Repository

Create a custom repository where we can set our custom queries and we will extend this interface in the main @Repository interface. The goal is to keep the repository access centralized in one single place.

For the custom query we use the MongoTemplate helper class to execute our query that will come with the filter.

With this class you can:

Perform CRUD operations (Create, Read, Update, Delete)
Execute queries
Work with indexes
Handle collections
Convert between Java objects and MongoDB documents (using MappingMongoConverter)

In this example we used the .regex to find an specific word inside both "name" and "summary" fields. I suggest you try for yourself to explore the diverse possibilities that the Query class can offer.