def join_in(self, *objs):
        """
        Create a join condition, connect A and C
        """
        if not objs:
            return self.table.c[self.fielda]!=self.table.c[self.fielda]
        else:
            keys = get_objs_columns(objs, self.reference_fieldname)
            return (self.table.c[self.fielda] == self.model_class.c[self.reversed_fieldname]) & (self.table.c[self.fieldb].in_(keys))
Create a join condition, connect A and C