def members
      NippoCore::User.joins(:group_member_relations)
                     .where(nippo_core_group_member_relations: {group_id: self.id})
                     .where.not(nippo_core_group_member_relations: {accepted_at: nil})
    end
def members
      NippoCore::User.joins(:group_member_relations)
                     .where(nippo_core_group_member_relations: {group_id: self.id})
                     .where.not(nippo_core_group_member_relations: {accepted_at: nil})
    end
TODO: implement test