Refs #14030 -- Removed more backwards compatiblity for old-style aggregates.

                    before_resolved = self.get_source_expressions()[index]

                    name = before_resolved.name if hasattr(before_resolved, 'name') else repr(before_resolved)

                    raise FieldError("Cannot compute %s('%s'): '%s' is an aggregate" % (c.name, name, name))

        c._patch_aggregate(query)  # backward-compatibility support

        return c

    @property

    def get_group_by_cols(self):

        return []

    def _patch_aggregate(self, query):

        """

        Helper method for patching 3rd party aggregates that do not yet support

        the new way of subclassing. This method will be removed in Django 1.10.

        add_to_query(query, alias, col, source, is_summary) will be defined on

        legacy aggregates which, in turn, instantiates the SQL implementation of

        the aggregate. In all the cases found, the general implementation of

        add_to_query looks like:

        def add_to_query(self, query, alias, col, source, is_summary):

            klass = SQLImplementationAggregate

            aggregate = klass(col, source=source, is_summary=is_summary, **self.extra)

            query.aggregates[alias] = aggregate

        By supplying a known alias, we can get the SQLAggregate out of the

        aggregates dict, and use the sql_function and sql_template attributes

        to patch *this* aggregate.

        """

        if not hasattr(self, 'add_to_query') or self.function is not None:

            return

        placeholder_alias = "_XXXXXXXX_"

        self.add_to_query(query, placeholder_alias, None, None, None)

        sql_aggregate = query.aggregates.pop(placeholder_alias)

        if 'sql_function' not in self.extra and hasattr(sql_aggregate, 'sql_function'):

            self.extra['function'] = sql_aggregate.sql_function

        if hasattr(sql_aggregate, 'sql_template'):

            self.extra['template'] = sql_aggregate.sql_template

class Avg(Aggregate):

    function = 'AVG'