Spring Boot之JPA杂记Inner Join

只为了实现一个很简单的操作：

select t1.id,t1.studentname,t2.gradename
from student t1
inner join grade t2
on t1.gradeid=t2.id

先创建了两个Entity

@Entity
public class Grade {
	@Id
	@GeneratedValue
	private Long id;
	private String gradename;
	//省略set get
	public Grade(Long id, String gradename) {
		super();
		this.id = id;
		this.gradename = gradename;
	}
}

@Entity
public class Student {
	@Id
	@GeneratedValue
	private Long id;
	private Long gradeid;
	private String studentname;
	//省略set get
	public Student(Long id, Long gradeid, String studentname) {
		super();
		this.id = id;
		this.gradeid = gradeid;
		this.studentname = studentname;
	}
}

因为返回的结果有id,studentname,gradename与这两个Entity都不同，所以又新建了一个StudentResult的POJO

public class StudentResult {
	Long id;
	String studentname, gradename;
	public StudentResult(Long id, String studentname, String gradename) {
		super();
		this.id = id;
		this.studentname = studentname;
		this.gradename = gradename;
	}
}

数据库数据如下

↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓疯狂尝试↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓

尝试一

repository

public interface StudentRepository extends JpaRepository {
	@Query(nativeQuery = true,
               value = "select t1.id,t1.studentname,t2.gradename from student t1 inner join grade t2 on t1.gradeid=t2.id")
	List getStudentResults();
}

controller

@RestController
@RequestMapping("/student")
public class StudentController {
	@Autowired
	StudentRepository studentRepository;
	@RequestMapping("/result")
	public List getStudentResults() {
		List studentResults = studentRepository.getStudentResults();
		return studentResults;
	}
}

Failed to convert from type [java.lang.Object[]] to type [cn.labsys.pmsys.entity.StudentResult]

2018-03-15 13:56:51.769 ERROR 9484 --- [nio-9090-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.Object[]] to type [cn.labsys.pmsys.entity.StudentResult] for value '{1, 1年级小学生A, 一年级}'; nested exception is org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.math.BigInteger] to type [cn.labsys.pmsys.entity.StudentResult]] with root cause

org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.math.BigInteger] to type [cn.labsys.pmsys.entity.StudentResult]

主要意思就是没有办法从Object[]转成StudentResult。

那如果直接返回Object[]呢。代码如下：

repository

@Query(nativeQuery = true, value = "select t1.id,t1.studentname,t2.gradename from student t1 inner join grade t2 on t1.gradeid=t2.id")
	List