@classmethod<EOL><INDENT>def id_field_name(cls):<DEDENT>
if cls._cache_id_field_name is None:<EOL><INDENT>pk_names = cls.pk_names()<EOL>if len(pk_names) == <NUM_LIT:1>:<EOL><INDENT>cls._cache_id_field_name = pk_names[<NUM_LIT:0>]<EOL><DEDENT>else:  <EOL><INDENT>raise ValueError(<EOL>"<STR_LIT>"<EOL>.format(classname=cls.__name__)<EOL>)<EOL><DEDENT><DEDENT>return cls._cache_id_field_name<EOL>
If only one primary_key, then return it. Otherwise, raise ValueError.

:rtype: str