现有用户实体，相关代码如下：

 @Entity
@Table(name = "t_users")
public class User extends Model {
    @Id
    @Column(name = "users_id")
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long userId;
    ...
}

以及好友关系实体，相关代码如下：

 @Entity
@Table(name = "t_relationship")
public class Relationship extends Model {
    @Id
    @Column(name = "rs_id")
    public Long relationshipId;

    @Column(name = "rs_fromuser", nullable = false)
    public User fromUser;

    @Column(name = "rs_touser", nullable = false)
    public User toUser;
    ...
}

从SQL书写来考虑，只需要在关系实体的两个Column上添加外键约束即可。但却不知道在JPA中应该怎么完成。

我个人数据库相关的知识比较欠缺，在这两个实体是不是多对多关系上存在疑问。
一个用户可以对应多个好友关系，一个好友关系对应 固定的两个用户 。应该说是n..2这种关系吗？

已经尝试使用 @ManyToMany 注解，但是我使用的Play 2框架和Ebean没有生成相对应的SQL语句。

 @Entity
@Table(name = "t_users")
public class User extends Model {
    @Id
    @Column(name = "users_id")
    @GeneratedValue(strategy = GenerationType.AUTO)
    @ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)
    @JoinTable(name = "users_relationship", joinColumns = {
            @JoinColumn(name = ("user_id"), referencedColumnName = "userId"),
            @JoinColumn(name = ("rs_id"), referencedColumnName = "relationshipId")
    })
    public Long userId;
    ...
}

 @Entity
@Table(name = "t_relationship")
public class Relationship extends Model {
    @Id
    @Column(name = "rs_id")
    public Long relationshipId;

    @Column(name = "rs_fromuser", nullable = false)
    @ManyToMany(targetEntity = User.class, mappedBy = "userId")
    public User fromUser;

    @Column(name = "rs_touser", nullable = false)
    @ManyToMany(targetEntity = User.class, mappedBy = "userId")
    public User toUser;
    ...
}

生成的SQL：

 create table t_relationship (
  rs_id                     bigint auto_increment not null,
  #这里缺少了两个user id列
  rs_makedate               datetime not null,
  rs_accepted               tinyint(1) default 0 not null,
  constraint pk_t_relationship primary key (rs_id))
;

补充：

后来注意到我需要映射的是User类而非User类的集合，在这里我大概是产生了一定的误解，需要搭配Play相关的内容来检查一下。