code: func (db *DB) schemaFieldKey(name, fieldType string) (key []byte, err error) {
  	if name == "" {
  		return nil, errors.New("field name can not be blank")
  	}
  	if fieldType == "" {
  		return nil, errors.New("field type can not be blank")
  	}
  	s, err := db.getSchema()
  	if err != nil {
  		return nil, err
  	}
  	var found bool
  	for n, f := range s.Fields {
  		if n == name {
  			if f.Type != fieldType {
  				return nil, fmt.Errorf("field %q of type %q stored as %q in db", name, fieldType, f.Type)
  			}
  			break
  		}
  	}
  	if !found {
  		s.Fields[name] = fieldSpec{
  			Type: fieldType,
  		}
  		err := db.putSchema(s)
  		if err != nil {
  			return nil, err
  		}
  	}
  	return append([]byte{keyPrefixFields}, []byte(name)...), nil
  }
docs: // schemaFieldKey retrives the complete LevelDB key for
  // a particular field form the schema definition.