java stream group by 对结果 value 进行二次 map 示例

var result = societyMemberService.list(new LambdaQueryWrapper<SocietyMember>()
                .eq(SocietyMember::getSocietyId, societies.stream().map(x -> x.getId()))
                .in(SocietyMember::getStudentUuid, students))
        .stream().collect(Collectors.groupingBy(x -> x.getStudentUuid(), Collectors.mapping(x -> x.getSocietyId(), Collectors.toList())));