Fixed #23085: Better error message for PostGIS 1.5/bad custom fields

        old_type = old_db_params['type']

        new_db_params = new_field.db_parameters(connection=self.connection)

        new_type = new_db_params['type']

        if old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and old_field.rel.through._meta.auto_created and new_field.rel.through._meta.auto_created):

        if (old_type is None and old_field.rel is None) or (new_type is None and new_field.rel is None):

            raise ValueError("Cannot alter field %s into %s - they do not properly define db_type (are you using PostGIS 1.5 or badly-written custom fields?)" % (

                old_field,

                new_field,

            ))

        elif old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and old_field.rel.through._meta.auto_created and new_field.rel.through._meta.auto_created):

            return self._alter_many_to_many(model, old_field, new_field, strict)

        elif old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and not old_field.rel.through._meta.auto_created and not new_field.rel.through._meta.auto_created):

            # Both sides have through models; this is a no-op.