code: func (fr FieldRow) MarshalJSON() ([]byte, error) {
  	if fr.RowKey != "" {
  		return json.Marshal(struct {
  			Field  string `json:"field"`
  			RowKey string `json:"rowKey"`
  		}{
  			Field:  fr.Field,
  			RowKey: fr.RowKey,
  		})
  	}
  	return json.Marshal(struct {
  		Field string `json:"field"`
  		RowID uint64 `json:"rowID"`
  	}{
  		Field: fr.Field,
  		RowID: fr.RowID,
  	})
  }
docs: // MarshalJSON marshals FieldRow to JSON such that
  // either a Key or an ID is included.