'Frozen set of all fields names (should only be used for many-one and many-many fields)'

@read_api
def all_field_names(self, field):

    if (field == u'formats'):
        return frozenset(self.fields[field].table.col_book_map)
    try:
        return frozenset(self.fields[field].table.id_map.itervalues())
    except AttributeError:
        raise ValueError((u'%s    is    not    a    many-one    or    many-many    field' % field))
