JPA many to many with extra column

Question

I have a following problem that I need to solve. The core issues is that I want to add additional column into JoinTable for ManyToMany relation in JPA. In my case I have fol

Answer 1

One technique that is useful when creating the many-to-many mapping class entity is to attribute the id's in the class along with @ManyToOne designation which makes this class act as the composite key class:

@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;
  }
}

This allows you to have the composite primary key defined within the same class without having to have a separate primary key class. You also need to make the class serializable.

Answer 2

Use list instead of set, together with the @OrderColumn annotation and JPA will automatically take care of the order:

@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;
    }

}

Generated DDL (using hibernate and 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 3

I would try to avoid using a List unless you allow duplicates.

There is a @OrderColumn annotation that automatically does this. Have you tried it?

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

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