多对一关系删除子对象时会抛出异常

我正在使用 JPA 进行多对一关系。从子表中删除子对象时会抛出异常。

下面是我的代码：

项目.java

  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  @Column(name="id")
  private int id;
  @Column(name="projectName")
  private String projectName;
  @Column(name="projectDesc")
  private String projectDesc; 

  @ManyToOne(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
  @JoinColumn(name="companyId")

公司.java

  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  @Column(name="id")
  private int id;
  @Column(name="compName")
  private String compName;
  @Column(name="address")
  private String address;

下面是插入代码：

InserAction.java

public static void main(String[] args) {
    Company comp2 = new Company();
    comp2.setCompName("IBM");
    comp2.setAddress("Bangalore");

    Project pro2 = new Project();
    pro2.setProjectName("Management System");
    pro2.setProjectDesc("System");
    pro2.setCompany(comp2);
    EntityManager entityManager = EntityManagerUtil.getEmf().createEntityManager(); 
    try{
      EntityTransaction entr = entityManager.getTransaction();
      entr.begin();
      entityManager.persist(pro2);
      entr.commit();
    }
 }

删除操作.java

EntityManager entityManager = EntityManagerUtil.getEmf()
            .createEntityManager();
    try {
        EntityTransaction entr = entityManager.getTransaction();
        entr.begin();       
        Project project = entityManager.find(Project.class,5);
        entityManager.remove(project);      

        entr.commit();
      }

例外的是

Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails (`prabha`.`project`, CONSTRAINT `FK_project_companyId` FOREIGN KEY (`companyId`) REFERENCES `company` (`id`))
Error Code: 1451
  Call: DELETE FROM company WHERE (id = ?)
  bind => [1 parameter bound]
  Query: DeleteObjectQuery(com.demo.manytoone.Company@301db5ec)

从项目表中删除项目对象时，它会抛出上述异常，我该如何克服这个问题。

 @ManyToOne(cascade={CascadeType.MERGE, CascadeType.PERSIST}, fetch=FetchType.EAGER)

上面的代码将解决您的问题。如果您观察注释 ManyToOne 的源代码，它有一个级联类型数组，因此您可以映射多个级联类型