如何在JPA中添加用户实体和好友关系实体的外键

Question

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。