Hibernate how to map set to an entity where I have two keys

I have two entitys A,B where A is one and B is many

A:

  @Entity
  @Table(name = "A") 
   class A...{
    @Id
@Column(name = "A2id",nullable = false)
private Integer A2id;

    @Id
@Column(name = "A1id",nullable = false)
private Integer A1id;

    @OneToMany(mappedBy="a")
private Set<B> Bset;

   //get's set's and public c'tor

}

B looks like this

@Entity
@Table(name = "B")
public class B implements Serializable{

//id's
@Id
@Column(name = "B1id", nullable = false)
private Integer B1id;
@Id
@Column(name = "B2id",nullable = false)
private Integer B2id;

@ManyToOne
@JoinColumn(name="B2id",nullable = false)
private A a;

//get's开发者_StackOverflow社区 set's and public c'tor
 }

and the hibernate shouts on me :

A Foreign key refering com..A from com..B has the wrong number of column. should be 2

what is wrong there? please help me

---

I found out my how to solve this issue :

using :

@ManyToOne
@JoinColumns( {
    @JoinColumn(name = "a1",  unique = false, nullable = false, insertable = false, updatable = false),
    @JoinColumn(name = "a2",  unique = false, nullable = false, insertable = false, updatable = false) })
private A a;