def joined_groups
      NippoCore::Group.joins(:group_member_relations)
                      .where(nippo_core_group_member_relations: {user_id: id})
                      .where.not(nippo_core_group_member_relations: {accepted_at: nil})
    end
def joined_groups
      NippoCore::Group.joins(:group_member_relations)
                      .where(nippo_core_group_member_relations: {user_id: id})
                      .where.not(nippo_core_group_member_relations: {accepted_at: nil})
    end
TODO: implement test