Writing Complex MongoDB Aggregation Operations in Java (Spring Mongo Template)

Writing complex MongoDB aggregation operations in Java using Spring’s Mongo Template doesn’t have to be complicated. By using a custom converter class, you can easily translate JSON-based MongoDB queries into Java code, making your code more maintainable and easier to understand. This approach is particularly useful for complex queries with multiple stages, allowing you to leverage the full power of MongoDB’s aggregation framework within your Java applications. Lets see how?

Example MongoDB Aggregation Operation

db.users.aggregate(
  [
    { $project : { month_joined : { $month : "$joined" } } } ,
    { $group : { _id : {month_joined:"$month_joined"} , number : { $sum : 1 } } },
    { $sort : { "_id.month_joined" : 1 } }
  ]
)

Converting the Aggregation to Spring MongoTemplate Java Code

Step 1: Create a Converter Class

	import org.bson.Document;
	import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
	import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;

	public class JsonAggregationOperation implements AggregationOperation {
	
	    private String jsonOperation;

	    public JsonAggregationOperation(String jsonOperation) {
		this.jsonOperation = jsonOperation;
	    }

	    @Override
	    public Document toDocument(AggregationOperationContext context) {
		return context.getMappedObject(Document.parse(jsonOperation));
	    }

	}

This class takes a JSON string representing a MongoDB aggregation stage and converts it into a Document that can be used by the MongoTemplate.

Step 2: Use the Converter Class in Your Service

Next, use this converter class in your service to execute the aggregation operation:

	import org.springframework.beans.factory.annotation.Autowired;
	import org.springframework.data.mongodb.core.MongoTemplate;
	import org.springframework.data.mongodb.core.aggregation.Aggregation;
	import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
	import org.springframework.stereotype.Service;
	import java.util.*;

	@Service
	public class MongoService {

	    @Autowired
	    MongoTemplate mongoTemplate;

	    public void complexAggregationOperation() {
		 List<AggregationOperation> aggOps = new ArrayList<>();
		 aggOps.add(new JsonAggregationOperation("{ $project : { month_joined : { $month : "$joined" } } }"));
		 aggOps.add(new JsonAggregationOperation("{ $group : { _id : {month_joined:"$month_joined"} , number : { $sum : 1 } } }"));
		 aggOps.add(new JsonAggregationOperation("{ $sort : { "_id.month_joined" : 1 } }"));

		 Aggregation aggregation = Aggregation.newAggregation(aggOps)
			.withOptions(Aggregation.newAggregationOptions().allowDiskUse(true).build());
		 List<Object> records = mongoTemplate.aggregate(aggregation, "users", Object.class).getMappedResults();
	    }
	}

That’s it. By using the above custom converter class, you can easily translate JSON-based MongoDB queries into Java code, making your code more maintainable and easier to understand.