有没有办法删除@OneToMany关系中的子实体（使用JoinTable），而无需获取完整的集合？ [英] Is there a way to delete a child entity in a @OneToMany relationship (with a JoinTable) without having to fetch the full collection?

问题描述

  public class 
 
 Contract implements Serializable {
 
 @Id 
 @GeneratedValue（strategy = GenerationType.AUTO）
 @Column（name =id）
 private Long id; 
 
 @版本
 @Column（name =version）
私有整数版本; 
 
私人字符串编号; 
私有字符串卷; 
 
 @OneToMany（cascade = CascadeType.REMOVE）
 @JoinTable（joinColumns = @JoinColumn（name =contract_id，referencedColumnName =id），inverseJoinColumns = @JoinColumn（name = attachment_id，referencedColumnName =id））
 private List< Attachment>附件; 
 
 
 public class Attachment implements Serializable {
 
 @Id 
 @GeneratedValue（strategy = GenerationType.AUTO）
 @Column（name =id）
私人长ID; 
 
 @版本
 @Column（name =version）
私有整数版本; 
 
 @Lob 
 @Basic（fetch = FetchType.LAZY）
 @Column（length = 2147483647）
 private byte [] contents; 
 
 
私人字符串名称; 
} 
  

根据我的需求/设计，我正在使用连接表。 p>

如果我想从合同中删除一个附件，我需要加载合同，然后遍历所有附件，直到找到想要删除的附件从列表中。

虽然这是功能性的，但它需要很多数据库通信。如果附件列表很长，并且包含大量内容，它还需要很大的带宽。

有没有其他办法可以删除它？如果我试图直接删除附件（例如：Attachment.findById（）。delete（）），它将因FK关系而失败 - 不是吗？ （我还没有尝试过，但我怀疑它）。

另外，如果我有一个非常大的附件列表，逐个遍历它们，直到找到正确的附件也不是非常有效。 b
$ b

JPA是否提供其他更好的解决方案？解决方案

- 你可以为连接表创建一个实体类。

你必须在 @JoinTable 注解（ name 属性），可以说 ContractAttachment 。然后你可以创建实体：

$ $ p $ @Entity（name =ContractAttachment）//注意同名的表
@IdClass（ContractAttachmentId.class）
public class ContractAttachment实现Serializable {

static final long serialVersionUID = 1L;

@Id
@ManyToOne
@JoinColumn（name =contract_id）//相同的列映射
私有合约;

@Id
@ManyToOne（cascade = CascadeType.REMOVE）
@JoinColumn（name =attachment_id）//列的相同映射
私有附件附件;

//你还必须在这里覆盖equals和hashcode方法


$ / code>

类 ContractAttachmentId 应该如下所示：



  public class ContractAttachmentId实现Serializable {
私人长期合约; //注意相同的字段名称
私有长附件; 
 
 //这个类也应该实现hashcode并且等于
} 
  

现在，您可以删除连接表中的单个条目，甚至还可以删除附件对象。

  ContractAttachment ca = em .createQuery（从ca选择ca+ 
其中ca.contract =：contract和ca.attachment =：attachment）
 .setParameter（contract，selectedContract）
。 setParameter（attachement，selectedAttachment）
 .getSingleResult（）; 
 em.remove（ca）; 
  




I've got the following classes/relationship (getters & setters not displayed, but present):

public class Contract implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;

    @Version
    @Column(name = "version")
    private Integer version;

    private String number;
    private String volume;

    @OneToMany(cascade=CascadeType.REMOVE)
    @JoinTable(joinColumns = @JoinColumn(name = "contract_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "attachment_id", referencedColumnName = "id"))
    private List<Attachment> attachments;
}

public class Attachment implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private Long id;

    @Version
    @Column(name = "version")
    private Integer version;

    @Lob
    @Basic(fetch=FetchType.LAZY)
    @Column(length=2147483647)
    private byte[] contents;


    private String name;
}

As per my needs/design, I am using a join table.

If I want to delete an attachment from the Contract, I need to load the Contract, and then loop through all the attachments until I find the one I want to remove and remove it from the list.

Although this is functional, it will require a lot of DB communication. If the list of attachments is long, and contains large contents, it will also require large bandwidth.

Is there any other way I can remove it? If I try to remove the attachment directly (ex: Attachment.findById().delete()), it will fail due to the FK relationship - won't it? (I haven't tried this yet, but I suspect it).

Additionally, if I have a very large list of attachments, iterating through them one by one until I find the correct one is not very efficient either.

Does JPA provide any other/better solution?

解决方案

There's one workaround solution I know - you can create an entity class for join table.

You will have to give a name to your join table within @JoinTable annotation (name attribute), lets say ContractAttachment. Then you can create entity:

@Entity(name = "ContractAttachment") // note the same name of table
@IdClass(ContractAttachmentId.class)
public class ContractAttachment implements Serializable {

    static final long serialVersionUID = 1L;

    @Id
    @ManyToOne
    @JoinColumn(name="contract_id") // same mappings for columns
    private Contract contract;

    @Id
    @ManyToOne(cascade = CascadeType.REMOVE)
    @JoinColumn(name="attachment_id") // same mappings for columns
    private Attachment attachment;

    // you will also have to override equals and hashcode methods here

}

The class ContractAttachmentId should look like:

public class ContractAttachmentId implements Serializable {
    private long contract; // note the same fields names
    private long attachment;

    // this class should also implement hashcode and equals
}

Now you can remove a single entry in join table and even cause attachment object to be deleted too.

ContractAttachment ca = em.createQuery("select ca from ContractAttachment ca " +
        "where ca.contract = :contract and ca.attachment = :attachment")
        .setParameter("contract", selectedContract)
        .setParameter("attachement", selectedAttachment)
        .getSingleResult();
em.remove(ca);

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