code: public Cursor aggregate(final List<? extends DBObject> pipeline, final AggregationOptions options) {
          return aggregate(pipeline, options, getReadPreference());
      }
docs: Method implements aggregation framework.
  
  @param pipeline operations to be performed in the aggregation pipeline
  @param options  options to apply to the aggregation
  @return the aggregation operation's result set
  @mongodb.driver.manual core/aggregation-pipeline/ Aggregation
  @mongodb.server.release 2.2