Minor improvement to proxy model handling

        result = []

        if opts is None:

            opts = self.query.model._meta

        # Skip all proxy to the root proxied model

        opts = opts.concrete_model._meta

        qn = self.quote_name_unless_alias

        qn2 = self.connection.ops.quote_name

        aliases = set()

        whereas column determination is a later part, and side-effect, of

        as_sql()).

        """

        # Skip all proxy models

        opts = self.model._meta.concrete_model._meta

        opts = self.model._meta

        root_alias = self.tables[0]

        seen = {None: root_alias}

        a parent of 'opts' or if it is None this method is a no-op.

        The 'alias' is the root alias for starting the join, 'seen' is a dict

        of model -> alias of existing joins.

        of model -> alias of existing joins. It must also contain a mapping

        of None -> some alias. This will be returned in the no-op case.

        """

        if model in seen:

            return seen[model]

            connection = (alias, int_opts.db_table, link_field.column, int_opts.pk.column)

            alias = seen[int_model] = self.join(connection, nullable=False,

                                                join_field=link_field)

        return alias

        return alias or seen[None]

    def remove_inherited_models(self):

        """