查询仅在其子项不为空时返回实体

Question

我有以下疑问：

  @Query("SELECT new de.hiqs.project.Project(project.id, project.name, project.isArchived, " +
          "project.customer.id, project.customer.name, project.customer.smallestTimeUnitEnabled, " +
          "project.customer.smallestTimeUnit, project.defaultDailyEntrySettings.id, " +
          "project.defaultDailyEntrySettings.performanceRecord, project.defaultDailyEntrySettings.startDateHour," +
          "project.defaultDailyEntrySettings.startDateMinute, project.defaultDailyEntrySettings.endDateHour, " +
          "project.defaultDailyEntrySettings.endDateMinute, project.defaultDailyEntrySettings.pause ) " +
          "FROM Project project")
  List<Project> findAllInOneRequest();

所以我有一个查询，它返回具有所选属性的所有项目，以便在一个请求中获取所有内容并避免 N+1 问题。如您所见，Project 与DefaultDailyEntrySettings 有关系（外键）。问题是并非每个项目都有DefaultDailyEntrySettings 子项（数据库中为空）。所以查询只返回有一个的projects。我希望它返回所有projects，如果defaultDailyEntrySettings不存在，则使用null初始化，而不是在没有defaultdailyEntrySettings的情况下不返回projects。有办法吗？

另一种选择是自动给每个project 一个带有空字段的defaultDailyEntrySettings，而不是外键为空，但我不知道如何使用注释来做到这一点。

Answer 1

也许是这样的？

@Query("SELECT new de.hiqs.project.Project(project.id, project.name, project.isArchived, " +
      "project.customer.id, project.customer.name, project.customer.smallestTimeUnitEnabled, " +
      "project.customer.smallestTimeUnit, project.defaultDailyEntrySettings.id, " +
      "project.defaultDailyEntrySettings.performanceRecord, project.defaultDailyEntrySettings.startDateHour," +
      "project.defaultDailyEntrySettings.startDateMinute, project.defaultDailyEntrySettings.endDateHour, " +
      "project.defaultDailyEntrySettings.endDateMinute, project.defaultDailyEntrySettings.pause ) " +
      "FROM Project project " +
      "WHERE EXISTS (SELECT 1 FROM project.defaultDailyEntrySettings)" +
      "UNION ALL " + 
      "SELECT new de.hiqs.project.Project(project.id, project.name, project.isArchived, " +
      "project.customer.id, project.customer.name, project.customer.smallestTimeUnitEnabled, " +
      "project.customer.smallestTimeUnit, null, " +
      "null, null," +
      "null, null, " +
      "null, null ) " +
      "FROM Project project " +
      "WHERE NOT EXISTS (SELECT 1 FROM project.defaultDailyEntrySettings)")
List<Project> findAllInOneRequest();