Refs #14030 -- Renamed CombinableMixin to Combinable

from django.utils.functional import cached_property

class CombinableMixin(object):

class Combinable(object):

    """

    Provides the ability to combine one or two objects with

    some connector. For example F('foo') + F('bar').

           in this query

         * reuse: a set of reusable joins for multijoins

         * summarize: a terminal aggregate clause

         * for_save: whether this expression about to be used in a save or update

        Returns: an ExpressionNode to be added to the query.

        """

                return agg, lookup

        return False, ()

    def prepare_database_save(self, field):

        return self

    def get_group_by_cols(self):

        if not self.contains_aggregate:

            return [self]

        return self

class ExpressionNode(BaseExpression, CombinableMixin):

class ExpressionNode(BaseExpression, Combinable):

    """

    An expression that can be combined with other expressions.

    """