JPA table with 2 primary key fields

Question

I have a table which contains only 2 fields. The table has a composite PK formed by these two fields.

When using Netbeans for creating entity beans from database, the entity bean is not created automatically like other tables that have more than 2 fields.

So I guess I need to create the entity bean myself. What is the best practice for creating this entity bean? Does it have to contain COMPOSITE KEY object or not?

Solution

I don't use NetBeans so I can't really say anything about its mapping tools.

For mapping a composite key, there are a couple of options. You can

• Define a separate @Embeddable object with the PK fields and use it as @EmbeddedId in your @Entity class

  @Embeddable
  public class MyCompositePK { 
      @Column
      private String fieldA;
      @Column
      private String fieldB;
  }
  @Entity 
  public class MyBean { 
      @EmbeddedId
      private MyCompositePK id;
      @Column
      private String fieldC;
  }
  

• Define a non-mapped POJO with the PK fields and use it as @IdClass in the @Entity.

  @Entity
  @IdClass(value=ClassAB.ClassABId.class)
  public class ClassAB implements Serializable {
      private String idA;
      private String idB;
  
      @Id
      @Column(name="ID_A")
      public String getIdA(){ return idA; }
      public void setIdA(String idA){ this.idA = idA; }
  
      @Id
      @Column(name="ID_B")
      public String getIdB(){ return idB; }
      public void setIdB(String idB){ this.idB = idB; }
  
      static class ClassABId implements Serializable {
          private String idA;
          private String idB;
  
          public String getIdA(){ return idA; }
          public void setIdA(String idA){ this.idA = idA; }
  
          public String getIdB(){ return idB; }
          public void setIdB(String idB){ this.idB = idB; }
  
          // implement equals(), hashcode()
      }
  }
  

  In this example ClassABId is a static inner class just for convenience.

These options are also explained in Pascal Thivent's excellent answer to this question: How to map a composite key with Hibernate?.

This related question discusses differences between these approaches: Which anotation should I use: @IdClass or @EmbeddedId. Notice the fields' declaration gets duplicated with the @IdClass approach.

Anyhow, I don't think there's an alternative to creating two classes. That's why I asked this question : Mapping a class that consists only of a composite PK without @IdClass or @EmbeddedId. It seems there's an hibernate-specific feature for this.

As a side note, if you've got control over the DB structure, you might also consider avoiding composite keys. There are some reasons to do so.

Comments

[Tom Anderson]

Given that the object contains only the two primary key fields, can it be its own IdClass?

[Xavi López]

@TomAnderson That's interesting. I've never tried. It seems that someone did, with no success. The spec doesn't seem to pronounce on this. Might depend on the implementation.

[Alvin Thompson]

@TomAnderson: I don't think so, because the ID class has to implement its equals() method based on just the composite key values.