code: func (fn *RandomFunction) Source() (interface{}, error) {
  	source := make(map[string]interface{})
  	if fn.field != "" {
  		source["field"] = fn.field
  	}
  	if fn.seed != nil {
  		source["seed"] = fn.seed
  	}
  	// Notice that the weight has to be serialized in FunctionScoreQuery.
  	return source, nil
  }
docs: // Source returns the serializable JSON data of this score function.