Mongodb $lookup in Spring data mongo

Not every "new" feature makes it immediately into abstraction layers such as spring-mongo.

So instead, all you need do is define a class that uses the AggregationOperation interface, which will instead take a BSON Object specified directly as it's content:

public class CustomAggregationOperation implements AggregationOperation {
    private DBObject operation;

    public CustomAggregationOperation (DBObject operation) {
        this.operation = operation;
    }

    @Override
    public DBObject toDBObject(AggregationOperationContext context) {
        return context.getMappedObject(operation);
    }
}

Then you can use in your aggregation like this:

Aggregation aggregation = newAggregation(
    match(
        Criteria.where("username").is("user001")
    ),
    new CustomAggregationOperation(
        new BasicDBObject(
            "$lookup",
            new BasicDBObject("from", "NewFeedContent")
                .append("localField","content.contentId")
                .append("foreignField", "_id")
                .append("as", "NewFeedContent")
        )
    )
)

Which shows the custom class mixed with the built in match() pipeline helper.

All that happens underneath each helper is that they serialize to a BSON representation such as with DBObject anyway. So the constructor here just takes the object directly, and returns it directly from .toDBObject(), which is the standard method on the interface that will be called when serializing the pipline contents.

Joining Two Collections with Spring Data MongoDB

Employee Class

class Employee {
    private String _id;
    private String name;
    private String dept_id;
}

Department Class

class Department {
    private String _id;
    private String dept_name;
}

Employee Result Class

public class EmpDeptResult {

    private String _id;
    private String name;
    private List<Object> departments;
}

EmployeeService Class

public class EmployeeService {

    @Autowired
    private MongoTemplate mongoTemplate;

    private Logger LOGGER = LoggerFactory.getLogger(EmployeeService.class);

    public void lookupOperation(){
    LookupOperation lookupOperation = LookupOperation.newLookup()
                        .from("Department")
                        .localField("dept_id")
                        .foreignField("_id")
                        .as("departments");

    Aggregation aggregation = Aggregation.newAggregation(Aggregation.match(Criteria.where("_id").is("1")) , lookupOperation);
        List<EmpDeptResult> results = mongoTemplate.aggregate(aggregation, "Employee", EmpDeptResult.class).getMappedResults();
        LOGGER.info("Obj Size " +results.size());
    }
}