options.py 文件源码

def _prepare(self, model):
        if self.order_with_respect_to:
            # The app registry will not be ready at this point, so we cannot
            # use get_field().
            query = self.order_with_respect_to
            try:
                self.order_with_respect_to = next(
                    f for f in self._get_fields(reverse=False)
                    if f.name == query or f.attname == query
                )
            except StopIteration:
                raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, query))

            self.ordering = ('_order',)
            if not any(isinstance(field, OrderWrt) for field in model._meta.local_fields):
                model.add_to_class('_order', OrderWrt())
        else:
            self.order_with_respect_to = None

        if self.pk is None:
            if self.parents:
                # Promote the first parent link in lieu of adding yet another
                # field.
                field = next(six.itervalues(self.parents))
                # Look for a local field with the same name as the
                # first parent link. If a local field has already been
                # created, use it instead of promoting the parent
                already_created = [fld for fld in self.local_fields if fld.name == field.name]
                if already_created:
                    field = already_created[0]
                field.primary_key = True
                self.setup_pk(field)
            else:
                auto = AutoField(verbose_name='ID', primary_key=True,
                        auto_created=True)
                model.add_to_class('id', auto)