Use $stdDevSamp or $stdDevPop with Spring Mongo

Question

I'd like to know how to implement a Standard Deviation aggregation Function to use in Spring Mongo Data.

I Know Mongo DB 3.2 has a Standard Deviation aggregation function, but It isn't available in Spring Data.

Could I use the Mongo's aggregation function?

Thanks.

Answer 1

There is a distinct difference between "not available" and "no implemented helper method", and that is the real case here. Just because there is no "helper" for implementing the $stdDevSamp or $stdDevPop operators, does not mean they cannot be used, as long as you are connecting to a MongoDB 3.2 instance of course.

All you really need is a custom class supporting the AggregationOperation interface, that will allow construction using DBObject:

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 that class in aggregation pipeline construction like so:

Aggregation aggregation = newAggregation(
    new CustomAggregationOperation(
        new BasicDBObject("$sample", new BasicDBObject("size",100))
    ),
    new CustomAggregationOperation(
        new BasicDBObject(
            "$group",
            new BasicDBObject("_id",null)
                .append("ageStdDev",new BasicDBObject("$stdDevSamp","$age"))
        )
    )
);

And that is the equivalent of the documentation example:

db.users.aggregate(
   [
      { "$sample": { "size": 100 } },
      { "$group": { "_id": null, "ageStdDev": { "$stdDevSamp": "$age" } } }
   ]
)

As an interface for AggregationOperation the class easily mixes with the implemented helpers:

Aggregation aggregation = newAggregation(
    // Using the match helper for the `$match` stage
    match(
        Criteria.where("age").gte(20).lte(50)
    ),
    // Mixed in with custom classes for the others
    new CustomAggregationOperation(
        new BasicDBObject("$sample", new BasicDBObject("size",100))
    ),
    new CustomAggregationOperation(
        new BasicDBObject(
            "$group",
            new BasicDBObject("_id",null)
                .append("ageStdDev",new BasicDBObject("$stdDevSamp","$age"))
        )
    )
);

So you can still use features even if there is no "buit in helper" to work out the BSON Object construction for you. You just do the construction yourself.