code: func (mc MongoClient) ReadingsByDeviceAndValueDescriptor(deviceId, valueDescriptor string, limit int) ([]contract.Reading, error) {
  	return mapReadings(mc.getReadingsLimit(bson.M{"device": deviceId, "name": valueDescriptor}, limit))
  }
docs: // Return a list of readings for a device filtered by the value descriptor and limited by the limit
  // The readings are linked to the device through an event