code: func (d DocvalueFields) Source() (interface{}, error) {
  	if d == nil {
  		return nil, nil
  	}
  	v := make([]interface{}, 0)
  	for _, f := range d {
  		src, err := f.Source()
  		if err != nil {
  			return nil, err
  		}
  		v = append(v, src)
  	}
  	return v, nil
  }
docs: // Source serializes the DocvalueFields into JSON.