java - 如何删除JPA @ManyToMany关系中的记录？

Question

我定义了一个单向ManyToMany关系，如下所示:

@Entity
@Table(name = "ODE_PROCESS_INSTANCE")
Public class ProcessInstanceDAOImpl extends OpenJPADAO implements ProcessInstanceDAO {
  @Id
  @Column(name = "ID")
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long _instanceId;

  @ManyToMany(targetEntity = PolicyAttachmentDAOImpl.class)
  private Collection<PolicyAttachmentDAO> _policyAttachments = new ArrayList<PolicyAttachmentDAO>();

  public ProcessInstanceDAOImpl() {
  }

  public Collection<PolicyAttachmentDAO> getPolicyAttachments(){
    return _policyAttachments;
  }

  public void setPolicyAttachments(Collection<PolicyAttachmentDAO> policyAttachments){
    _policyAttachments = policyAttachments;
    getEM().merge(this);
    getEM().flush();
  }
}

@Entity
@Table(name = "ODE_POLICY_ATTACHMENT")
public class PolicyAttachmentDAOImpl extends OpenJPADAO implements PolicyAttachmentDAO{
  @Id
  @Column(name = "POLICYATTACHMENT_ID")
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long _attachmentId;

  @Basic
  @Column(name = "POLICYATTACHMENT_NAME")
  private String _attachmentName;

  public PolicyAttachmentDAOImpl(String policyAttachmentName) {
    _attachmentName = policyAttachmentName;
  }

  public void deletePolicyAttachment(){
    //delete policy attachment DAO
getEM().remove(this);
getEM().flush();
  }
}

连接表已创建。问题是，当我从 ODE_POLICY_ATTACHMENT 中删除记录时，连接表不会更新。我该如何解决这个问题？

这就是我添加到ta的方式

Collection<PolicyAttachmentDAO> policyAttachments = new ArrayList<PolicyAttachmentDAO>();

attachmentDAO = new PolicyAttachmentDAOImpl("attachment_1");
policyAttachments.add(attachmentDAO);

attachmentDAO = new PolicyAttachmentDAOImpl("attachment_2");
policyAttachments.add(attachmentDAO);

attachmentDAO = new PolicyAttachmentDAOImpl("attachment_3");
policyAttachments.add(attachmentDAO);

//inst is an exisiting ProcessInstanceDAOImpl
inst.setPolicyAttachments(policyAttachments);

要从“ODE_PROCESS_INSTANCE”表中删除记录，我在 PolicyAttachmentDAOImpl 对象上调用方法 deletePolicyAttachment()。

Answer 1

连接表用于映射多对多关联。它让 Hibernate 知道某些附件与某些进程相关联。因此，如果您不希望在连接表中引用附件，则意味着您不再希望该附件链接到任何进程。所以答案很简单:找到其集合中包含附件的所有进程，并从其集合中删除附件:

process.getPolicyAttachments().remove(theAttachment);

旁注:你的命名很糟糕。在字段前面添加下划线不是标准约定，并且会强制您在所有查询中添加此前导下划线。你所拥有的不是 DAO，而是实体。 DAO 是用于查询和更新数据库的对象。实体是 DAO 返回的持久对象。并且每次更改进程的附件时都不应该合并和刷新。合并很少是必要的，刷新也几乎从不存在，无论如何这都不应该由实体完成，因为实体根本不应该访问 EM。