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。
解决方案
2014.4.20再次更新
虽然上次的方法已经能够解决问题,但在开发过程中累积了一些经验后发现那样的解决方法算不上好。
下面更新下我现在的方案。
// User model类@Entity@Table(name = "t_users")public class User extends Model { @Id @Column(name = "u_id") @GeneratedValue(strategy = GenerationType.AUTO) public Long userId; @OneToMany(fetch = FetchType.LAZY, mappedBy = "fromUser") public List<Relationship> followUsers; @OneToMany(fetch = FetchType.LAZY, mappedBy = "toUser") public List<Relationship> followers; ...}@Entity@Table(name = "t_relationship")public class Relationship extends Model { @Id @Column(name = "rs_id") public Long relationshipId; @JoinColumn(name = "rs_fromuser", nullable = false, updatable = false) @ManyToOne(optional = false) public User fromUser; @JoinColumn(name = "rs_touser", nullable = false, updatable = false) @ManyToOne(optional = false) public User toUser; ...}
以下为2014.3.18答案
问题经过探索后已经解决,但并没有完全理解其中的门道。关于JPA和ORM的基础知识不够扎实,希望能得到解惑。
就如前面所怀疑的,这并不是多对多关系,而是简单的一对一,不过是一个实体中有两个一对一关系罢了。
换用@OneToOne
注解,并且了解了一下其他相关的内容,修改为下面的样子。
// User model类@Entity@Table(name = "t_users")public class User extends Model { @Id @Column(name = "users_id") @GeneratedValue(strategy = GenerationType.AUTO) public Long userId; ...}// Relationship model类@Entity@Table(name = "t_relationship")public class Relationship extends Model { @Id @Column(name = "rs_id") public Long relationshipId; @JoinColumn(name = "rs_fromuser", referencedColumnName = "users_id", nullable = false, updatable=false) @OneToOne(optional = false, targetEntity = User.class) public User fromUserId; @JoinColumn(name = "rs_touser", referencedColumnName = "users_id", nullable = false, updatable=false) @OneToOne(optional = false, targetEntity = User.class) public User toUserId; ...}