code: func (ms *modelSpec) fieldIndexKey(fieldName string) (string, error) {
  	fs, found := ms.fieldsByName[fieldName]
  	if !found {
  		return "", fmt.Errorf("Type %s has no field named %s", ms.typ.Name(), fieldName)
  	} else if fs.indexKind == noIndex {
  		return "", fmt.Errorf("%s.%s is not an indexed field", ms.typ.Name(), fieldName)
  	}
  	return ms.name + ":" + fs.redisName, nil
  }
docs: // fieldIndexKey returns the key for the sorted set used to index the field identified
  // by fieldName. It returns an error if fieldName does not identify a field in the spec
  // or if the field it identifies is not an indexed field.