Question

我有一个我需要解决的问题。核心问题是我想在JPA中为JoinTable for ManyToMany关系添加额外的列。就我而言，我有以下实体。

主题是一个简单的实体，它有许多RemoteDocument（一个RemoteDocument可能被许多主题引用，因此它应该是ManyToMany关系）。此外，RemoteDocument实体是只读的，因为它可能只能从Oracle物化视图中读取，而且禁止更改此物化视图。所以我想存储与某些主题相关的RemoteDocuments的顺序。事实上，我可以使用其他实体做类似的事情：

@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。

但是我想拥有ManyToMany关系，但是在连接表中配置了额外的列

Answer 1

使用list而不是set，以及@OrderColumn注释，JPA将自动处理订单：

@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;

Answer 2

除非您允许重复，否则我会尽量避免使用List。

有一个@OrderColumn注释会自动执行此操作。你试过吗？

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

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

Answer 3

在创建多对多映射类实体时，一种有用的技术是将类中的id与@ManyToOne指定一起归属，这使得该类充当复合键类：

@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;
  }
}

这允许您在同一个类中定义复合主键，而不必拥有单独的主键类。您还需要使类可序列化。

JPA很多都有额外的列

3 个答案: