How do you Set Up a Many-to-Many Relationship with Junction Table using JPA/EclipseLink

Question

I have 2 tables:

Movies: movieID

Users: userID

These tables have a many to many relationship through the Queue table, with an additional attribut

Answer 1

First of all, as suggested by Mike Cornell, the EmbeddedId/Class is probably the easier-to-use choice. Nonetheless, to answer to your question and corrected code:

@IdClass(QueueItemPK.class)
@Entity
@Table(name="queue")
public class QueueItem
{
       @Id
       @ManyToOne(optional=false)
       @PrimaryKeyJoinColumn(name="movieID")
       private Movie movie;

       @Id
       @ManyToOne(optional=false)
       @PrimaryKeyJoinColumn(name="userID")
       private User user;

       @Basic
       private String listOrder;

       ...Getters/Setters...
}

public class QueueItemPK implements Serializable
{
       private static final long serialVersionUID = 1L;

       @Id
       @Column(name="movieID")
       private Integer movie;
       @Id
       @Column(name="userID")
       private Integer user;

       ...Getters/Setter...

       public int hashCode()
       {
           return (movie.getMovieID() + "|" + user.getUserID()).hashCode();
       }

       public boolean equals(Object obj)
       {
           if (obj == this) return true;
           if (obj == null) return false;
           if (!(obj instanceof QueueItemPK)) return false;
           QueueItemPK pk = (QueueItemPK) obj;
           return pk.movie == movie 
               && pk.user == user;
       }
}

As you can see it is neccessary that they have the type of the id's they've to match. not very beautiful, but working. And i suggest using generics for your Sets; makes ist easier to read and more secure to code.