Self ManyToMany with additional columns using JPA 2.0

I want to create kind of "Friendship" relation between Users models. I also need an additional columns for every friendship. I know I need to use joining class with composite primary key. Thats my User class

public class User implements Serializable {

@Id
@Basic(optional = false)
@Column(name = "id")
@GeneratedValue(strategy=GenerationType.IDENTITY)
private Integer id;

@OneToMany(mappedBy="friendA")
private Set<Friendship> friends;

I also have FriendshipId class

@Embeddable
public class FriendshipId implements Serializable {
private long friendAId; 
private long friendBId;
}

And finally Friendship class

@Entity
@IdClass(FriendshipId.class)
public class Friendship implements Serializable {


private Integer friendAId;

private Integer friendBId;

@Basic(optional = false)
@Column(name="date_added")
@Type(type="org.jadira.usertype.dateandtime.joda.PersistentDateTime")
private DateTime date_added;


@ManyToOne
@PrimaryKeyJoinColumn(name="friendAId", referencedColumnName="friendAId")
private User friendA;

@ManyToOne
@PrimaryKeyJoinColumn(name="friendBId", referencedColumnName="friendBId")
private User friendB;
}

This seems to work but database table generated in my database consists every ID twice - I guess one set is for primary key and second for foreign keys.

How can I make this PK unique - that Friendship of A to B will be rejected if there is B to A one?

Another question is about a design - is this a good way to achieve what I want to achieve? I me开发者_JAVA百科an using composite key instead of simply own pk for Friendship.

I guess in Friendship entity you added User entity twice (very fine) but why

private Integer friendAId;
private Integer friendBId;

again there? That could be cause of ID getting generated twice.