JPA ManyToMany一项字段的双向自引用

Question

我有一个实体用户。我需要一个字段中的ManyToMany双向关系User-User对。我该怎么办？

@Entity
@Table(name = "users")
public class User {
    @Id
    private int id;
    private Set<User> pairs;
}

我尝试过这样：

@ManyToMany
@JoinTable(name = "pairs", joinColumns = {
    @JoinColumn(name = "a"), 
    @JoinColumn(name = "b")
})
private Set<User> pairs;

得到下一个结果：org.hibernate.AnnotationException: A Foreign key refering com.calm.model.entity.User from com.calm.model.entity.User has the wrong number of column. should be 1

ddl产生的db scmema：

CREATE TABLE `users` (
    `id` int(11) NOT NULL,
    ...
     PRIMARY KEY (`id`)
)

和期望对表：

CREATE TABLE `pairs` (
    `a` int(11) NOT NULL, //user 1
    `b` int(11) NOT NULL, //user 2
    PRIMARY KEY (`a`,`b`)
)

预期的行为，例如：

SELECT b as id2 FROM pairs WHERE a = :id1
UNION 
SELECT a as id2 FROM pairs WHERE b = :id1