Question

我有一个多对多的关系，如下所示。

@Entity
@Table(name = "employees")
public class Employee {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "EMP_ID")
  private int id;
  
  private String firstName;
  private String lastName;

  @JoinTable(name = "employee_project_mapping", joinColumns = @JoinColumn(name = "EMPLOYEE_ID", referencedColumnName = "EMP_ID"), inverseJoinColumns = @JoinColumn(name = "PROJECT_ID", referencedColumnName = "PJT_ID"))
  @JsonManagedReference
  @ManyToMany(cascade = CascadeType.ALL)
  Set<Project> projects = new HashSet<Project>();
  .....
  .....
}



@Entity
@Table(name = "projects")
public class Project {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "PJT_ID")
  private int projectId;

  private String projectName;

  private int teamSize;

  @ManyToMany(mappedBy = "projects")
  @JsonBackReference
  private Set<Employee> emps = new HashSet<Employee>();
  .....
  .....
}

@Entity
@Table(name = "employee_project_mapping")
public class EmployeeProjectMapping {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "EMP_PJT_ID")
  private Integer empPjtId;

  @Column(name = "PROJECT_ID")
  private Integer projectId;

  @Column(name = "EMPLOYEE_ID")
  private Integer emploeeId;

  .....
  .....
}

但是当我尝试插入带有一组项目的员工对象时，它无法为列 EMP_PJT_ID 创建自动生成的 ID（这是映射表记录的 ID）。我不能使用 jpa 为映射表添加自动生成的 id 吗？

错误跟踪

Hibernate: insert into employees (emp_id, first_name, last_name) values (null, ?, ?)
Hibernate: insert into employee_project_mapping (employee_id, project_id) values (?, ?)
2021-04-22 23:34:25.973 ERROR 24126 --- [nio-8080-exec-9] o.h.engine.jdbc.spi.SqlExceptionHelper   : NULL not allowed for column "EMP_PJT_ID"; SQL statement:
insert into employee_project_mapping (employee_id, project_id) values (?, ?) [23502-200]
2021-04-22 23:34:25.975 ERROR 24126 --- [nio-8080-exec-9] 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.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement] with root cause

org.h2.jdbc.JdbcSQLIntegrityConstraintViolationException: NULL not allowed for column "EMP_PJT_ID"; SQL statement:
insert into employee_project_mapping (employee_id, project_id) values (?, ?) [23502-200]

Answer 1

多对多的映射应该是：

@Entity
@Table(name = "employees")
public class Employee {
...

  @OneToMany(mappedBy = "employee", cascade = CascadeType.ALL, orphanRemoval = true)
  Set<EmployeeProjectMapping> projects = new HashSet<EmployeeProjectMapping>();
...

    // Utility method to update both sides of the association
    public void addProject(Project project) {
        EmployeeProjectMapping empProject = new PersEmployeeProjectMappingonAddress( this, project );
        projects.add( empProject );
        project.getEmployees().add( empProject );
    }
}


@Entity
@Table(name = "projects")
public class Project {
...
   @OneToMany(mappedBy = "project", cascade = CascadeType.ALL, orphanRemoval = true)
   private Set<EmployeeProjectMapping> emps = new HashSet<Employee>();
...
}

@Entity
@Table(name = "employee_project_mapping")
public class EmployeeProjectMapping {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "EMP_PJT_ID")
  private Integer empPjtId;

  @Id
  @ManyToOne
  @JoinColumn(name = "PROJECT_ID")
  private Project project;

  @Id
  @ManyToOne
  @JoinColumn(name = "EMPLOYEE_ID")
  private Employee employee;

  .....
  .....
}

请务必检查 the example on the Hibernate ORM documentation 以了解所有详细信息。

@GeneratedValue 注释不适用于映射表

1 个答案: