JPA 多对多，带有额外列

Question

我有一个需要解决的问题。 核心问题是我想在 JPA 中的 ManyToMany 关系的 JoinTable 中添加额外的列。就我而言，我有以下实体。

Topic是一个简单的实体，它有许多RemoteDocument（一个RemoteDocument可能被许多Topic引用，因此它应该是ManyToMany关系）。 RemoteDocument 实体也是只读的，因为它只能从 Oracle 物化视图中读取，而且禁止对该物化视图进行任何更改。所以我想存储与某些主题相关的 RemoteDocuments 的顺序。事实上，我可以使用附加实体执行类似的操作：

@Entity
public class Topic {
 @Id
 private Long id;
 @Basic
 private String name;

    @OneToMany
 private Set<TopicToRemoteDocument> association;
}

@Entity
public class RemoteDocument {
 @Id
 private Long id;
 @Basic
 private String description;
}

@Entity
public class TopicToRemoteDocument {
 @OneToOne
 private Topic topic;
 @OneToOne
 private RemoteDocument remoteDocument;
 @Basic
 private Integer order;
}

在这种情况下，附加实体 TopicToRemoteDocument 帮助我用 OneToMany 替换 ManyToMany 关联并添加额外的字段顺序。

但我想要拥有多对多关系，但在连接表中配置了附加列

Answer 1

我会尽量避免使用 List 除非你允许重复。

有一个 @OrderColumn 注释可以自动执行此操作。你尝试过吗？

@Entity
public class Topic {
 @Id
 private Long id;
 @Basic
 private String name;

 @OneToMany
 @OrderColumn
 private Set<TopicToRemoteDocument> association;
}

Answer 2

创建多对多映射类实体时有用的一种技术是将类中的 id 与 @ManyToOne 指定一起赋予属性，这使得此类充当复合键类：

@Entity
@Table(name = "market_vendor")
public class MarketVendor implements Serializable 
{
  @Id
  @ManyToOne
  @JoinColumn(name = "market_id")
  private Market market;

  @Id
  @ManyToOne
  @JoinColumn(name = "vendor_id")
  private Vendor vendor;

  @Basic
  @Column(name="active")
  private boolean active;

  public MarketVendor(Market market, Vendor vendor, boolean active)
  {
    this.market = market;
    this.vendor = vendor;
    this.active = active;
  }
}

这允许您定义复合主键在同一个类中，无需有单独的主键类。您还需要使该类可序列化。

Answer 3

使用列表而不是集合，再加上@OrderColumn注释，JPA将自动处理顺序：

@MappedSuperclass
public class BaseEntity{

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

    public Long getId(){
        return id;
    }

    public void setId(final Long id){
        this.id = id;
    }

}

@Entity
public class Topic extends BaseEntity{

    @ManyToMany(mappedBy = "topics")
    @OrderColumn
    private List<Document> documents = new ArrayList<Document>();

    public List<Document> getDocuments(){
        return documents;
    }

    public void setDocuments(final List<Document> documents){
        this.documents = documents;
    }

}

@Entity
public class Document extends BaseEntity{

    @ManyToMany
    @OrderColumn
    private List<Topic> topics = new ArrayList<Topic>();

    public List<Topic> getTopics(){
        return topics;
    }

    public void setTopics(final List<Topic> topics){
        this.topics = topics;
    }

}

生成的DDL（使用hibernate和HSQL）：

create table Document (
    id bigint generated by default as identity (start with 1),
    primary key (id)
);

create table Document_Topic (
    documents_id bigint not null,
    topics_id bigint not null,
    topics_ORDER integer not null,
    documents_ORDER integer not null,
    primary key (documents_id, topics_ORDER)
);

create table Topic (
    id bigint generated by default as identity (start with 1),
    primary key (id)
);

alter table Document_Topic 
    add constraint FK343B5D0B481100B2 
    foreign key (documents_id) 
    references Document;

alter table Document_Topic 
    add constraint FK343B5D0B558627D0 
    foreign key (topics_id) 
    references Topic;