How to use $cond operation in Spring-MongoDb aggregation framework

Question

I have an aggregation pipeline which includes a project like this:

$project: {
  start: {
    $cond: {
      if: {
        $eq: ["$start", "EARLY"]
      },
      then: "$deltastart.start",
      else: "$deltastart.end"
    }
  },...
},...

which works fine in mongo shell. How to express this using the Aggregation framework in Spring-Mongodb? I have seen ProjectionOperationBuilder, ExpressionProjectionOperationBuilder types but not an example how to use them... any suggestions?

Answer 1

If using the current Spring Data release which has support for the $cond operator via the $project pipeline, then this can be converted to (untested):

import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
import static org.springframework.data.mongodb.core.aggregation.ConditionalOperators.Cond.*;
import org.springframework.data.mongodb.core.query.Criteria;

Cond condOperation = ConditionalOperators.when(Criteria.where("start").is("EARLY"))
                                    .thenValueOf("deltastart.start")
                                    .otherwise("deltastart.end");

Aggregation agg = newAggregation(project().and(condOperation).as("start"));
AggregationResults<MyClass> results = mongoTemplate.aggregate(agg, MyClass.class); 
List<MyClass> myList = results.getMappedResults();

---

For Spring-Data MongoDB version which do not have support for the $cond operator in the aggregation operation, there is a workaround which is to implement the AggregationOperation interface to take in a DBObject:

public class CustomProjectAggregationOperation implements AggregationOperation {
    private DBObject operation;

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

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

Then implement the $project operation as a DBObject in the aggregation pipeline that is the same as the one you have:

DBObject operation = (DBObject) new BasicDBObject(
    "$project", new BasicDBObject(
         "start", new BasicDBObject(
                "$cond", new Object[]{
                        new BasicDBObject(
                            "$eq", new Object[]{ "$start", "EARLY"}
                        ),
                        "$deltastart.start",
                        "$deltastart.end"
                 }
           )
     )
);

which you can then use in TypeAggregation:

TypedAggregation<CustomClass> aggregation = newAggregation(CustomClass.class,
    new CustomProjectAggregationOperation(operation)
);
AggregationResults<CustomClass> result = mongoTemplate.aggregate(aggregation, CustomClass.class);