How to persist an entity with Composite primary key

Question

Problem Statement:-
Entity A have many to one relation with Entity C
Entity B have many to one relation with Entity C
Entity C have a composite primary key of (Entity A PK & Entity B PK)
A --< C
B --< C
the entites are automatic generated by Dali tool
@Entity
public class C implements Serializable
     @EmbeddedId
     private C.PK pk;
     private int attribute;
     //Code
     @Embeddable
     public static class PK implements Serializable
          public int A_Id;
          public int B_Id;
          //Code
@Entity
public class A implements Serializable
     @Id
     private int AId;
     @OneToMany
     private Set<C> C_Collection;
     //Code
@Entity
public class B implements Serializable
     @Id
     private int BId;
     @OneToMany
     private Set<C> C_Collection;
     //Code
i made a session Bean which add a new entity C
Session
     A a = em.find(A.class, AId);
     B b = em.find(B.class, BId);
     C c = new C();
     C.PK pk = new C.PK();
     pk.AId(a.getID());
     pk.BId(b.getID());
     c.setPk(pk);
     c.setA(a);
     c.setB(b);
     c.setAttribute(12);
     em.persist(c);
when running this code an exception is raised
Exception [TOPLINK-4002] (Oracle TopLink Essentials - 2006.8 (Build 060830)): oracle.toplink.essentials.exceptions.DatabaseException
Internal Exception: com.mysql.jdbc.exceptions.MySQLSyntaxErrorException: Unknown column 'A_ID' in 'field list'Error Code: 1054
Call:INSERT INTO C (attribute, Aid, A_ID, B_ID, Bid) VALUES (?, ?, ?, ?, ?)
     bind => [2, 6, 6, 1, 1]
this is logic as Table "C" has only 3 coloumns only "attribute" , "Aid" , "Bid"
but the mapping is different as it adds the composite PK attributes to the insert statement
if there is a sample code or how to solve this issue it would be great

Answer 1

Hello Juwen,
The problem is you are registering an object, assigning it a null pk, and then commiting the uow/transaction. TopLink uses registered objects to keep track of changes you make inorder to persist those changes on commit. So the simple fix is to not commit the UnitOfWork - call release() on it instead.
Another solution is to use the session copyObject api. The simple form that only takes an object will work similar to registering the object as it will copy all persistent attributes but it will leave the primary key null. You can also use this method to specify a copyPolicy to customize the process. Using this method will be a bit more efficient, since a UOW makes both a working copy and a back up copy of objects registered, inorder to keep track of changes. Using the copyObject api will only make a single copy.
Best Regards,
Chris