Spring数据JPA接口和基于类的投影对嵌入键的不同字段不起作用

Question

select DISTINCT(ID), NAME, DEPARTMENT from EMPLOYEE;

我试图使用基于接口和基于类的Spring data JPA投影，但这些方法似乎都行不通。基于接口的投影给出了我无法取消代理的代理列表。在基于类的投影的构造函数中使用DISTINCT是不可能的。

@Entity
@Table(name = "EMPLOYEE")
public class Employee {
   @EmbeddedId
   private EmployeeKey key;

   @Column(name = "NAME")
   private String name;

   @Column(name = "DEPARTMENT")
   private String department;

   @Coulmn(name = "AGE")
   private Integer age;

   //Getter/Setters/Constructors
}

@Embeddable
public class EmployeeKey {
   @Column(name = "ID")
   String id;

   @Column(name = "REGNO")
   String regNo;

   //Getter/Setters/Constructors
}

@Repository
public interface EmployeRepository extends JpaRepository<Employee, EmployeeKey>{
   @Query(value = "select distinct(emp.key.id), emp.name, emp.department from Employee emp")
   List<EmployeeInterfaceProjection> findUsingInterfaceProjection();

   @Query(value = "select distinct(emp.key.id) as empId, emp.name as empName, emp.department as empDepartment from Employee emp")
   List<EmployeeClassProjection1> findUsingClassProjection1();

   @Query(value = "select new com.path.to.EmployeeClassProjection2(emp.key, emp.name, emp.department) from Employee emp")
   List<EmployeeClassProjection2> findUsingClassProjection2();

   @Query(value = "select distinct(emp.key.id) as empId, emp.name as empName, emp.department as empDepartment from Employee emp")
   List<Object[]> findUsingObjectProjection();
}

public interface EmployeeInterfaceProjection{
   EmployeeKeyInterfaceProjection getKey();
   String getName();
   String getDepartment();

   interface EmployeeKeyInterfaceProjection{
      String getId();
   }
}

public class EmployeeClassProjection1{
   private String empId;
   private String empName;
   private String empDepartment;

   //Getters/Setters, Constructors, Hashcode, Equals
}

public class EmployeeClassProjection2{
   private EmployeeKey key;
   private String name;
   private String department;

   //Getters/Setters, Constructors, Hashcode, Equals
}

每种方法所面临的问题

findUsingInterfaceProjection（）

findUsingObjectProjection（）

这个方法很管用，但看起来是一个很粗糙的方法。我希望使用JPA的预测。

Answer 1

这就是我最终成功的原因。我在查询中使用了与projection中的getter方法名匹配的列别名（alias=name，如果projection具有getName（））。不需要使用内部接口EmployeeKeyInterfaceProjection。

public interface EmployeeInterfaceProjection{
   String getId();
   String getName();
   String getDepartment();
}


@Query(value = "select distinct(emp.key.id) as id, emp.name as name, emp.department as department from Employee emp")
List<EmployeeInterfaceProjection> findUsingInterfaceProjection();

基于接口的投影在AbstractJPAQuery$TupleConverter$TupleBackedMap内部工作。另外，在不使用列别名的情况下，我为getId()、getName()、getDepartment（）中的每一个都得到了null。使用别名解决了这个问题。

谢谢@Ho Wai Chan给我指明了正确的方向。