为什么查询结果映射为 COUNT(*) 为空值？

Question

interface TaskReviewRepository : CoroutineCrudRepository<TaskReview, Long> {
  @Query(
    """
    SELECT task_id, AVG(rating) as avgRating, COUNT(*) as reviewsCount
    FROM task_review
    WHERE task_id IN (:taskIds)
    GROUP BY task_id
  """
  )
  fun ratingSummaryWhereTaskIdIn(taskIds: Set<Long>): Flow<TaskReviewService.RatingSummaryWithTaskId>
}

data class RatingSummaryWithTaskId(
  val taskId: Long,
  val avgRating: Double?,
  val reviewsCount: Long,
)

当我在数据库中手动运行这个查询时

SELECT task_id, AVG(rating) as avgRating, COUNT(*) as reviewsCount
FROM task_review
WHERE task_id IN (1)
GROUP BY task_id

结果是：

task_id  | avgRating  | reviewsCount
      1  |   5.00000  |            1

但是当我使用来自服务的存储库时，我得到：

2021-06-30 09:22:33.240 DEBUG 7529 --- [     parallel-3] io.r2dbc.postgresql.PARAM                : Bind parameter [0] to: 1
2021-06-30 09:22:33.246 DEBUG 7529 --- [     parallel-3] io.r2dbc.postgresql.QUERY                : Executing query: 
    SELECT task_id, AVG(rating) as avgRating, COUNT(*) as reviewsCount
    FROM task_review
    WHERE task_id IN ()
    GROUP BY task_id
// ...
org.springframework.data.mapping.model.MappingInstantiationException: Failed to instantiate com.example.tasks.TaskReviewService$RatingSummaryWithTaskId using constructor fun <init>(kotlin.Long, kotlin.Double?, kotlin.Long): com.example.tasks.TaskReviewService.RatingSummaryWithTaskId with arguments 1,null,null
    at org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator$EntityInstantiatorAdapter.createInstance(ClassGeneratingEntityInstantiator.java:246) ~[spring-data-commons-2.5.1.jar:2.5.1]
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
// ...
Caused by: java.lang.IllegalArgumentException: Parameter reviewsCount must not be null!

为什么映射器使用 null 作为第三个参数？ (com.example.tasks.TaskReviewService.RatingSummaryWithTaskId with arguments 1,null,null)

我的猜测是存储库本身使用 TaskReview 而不是 RatingSummaryWithTaskId 作为泛型，但据我所知，可以为映射定义任意 return 类型，并且它适用于其他存储库。

我正在使用 spring-boot 2.5.1 与 spring-boot-starter-data-r2dbc 和 io.r2dbc:r2dbc-postgresql。其他存储库按预期工作。

Answer 1

事实证明使用蛇形外壳而不是驼峰外壳效果很好：

SELECT task_id, AVG(rating) as avg_rating, COUNT(*) as reviews_count
FROM task_review
WHERE task_id IN (:taskIds)
GROUP BY task_id

因为不支持在选定列的别名中使用驼峰式大小写是意想不到的，所以我在这里提出了一个问题来跟进 spring 好人：https://github.com/spring-projects/spring-data-r2dbc/issues/616

为什么查询结果映射为 COUNT(*) 为空值？

Why is the query result mapped with a null value for COUNT(*)?

spring-boot

kotlin

kotlin-coroutines

r2dbc-postgresql