JPA：定义复杂的@OneToMany关系

Question

我定义了一组表t1，t2，... tN：

mysql>  desc t1;
+-------------+------------------+------+-----+---------+----------------+
| Field       | Type             | Null | Key | Default | Extra          |
+-------------+------------------+------+-----+---------+----------------+
| id          | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
(...)
+-------------+------------------+------+-----+---------+----------------+

(...)

mysql>  desc tN;
+-------------+------------------+------+-----+---------+----------------+
| Field       | Type             | Null | Key | Default | Extra          |
+-------------+------------------+------+-----+---------+----------------+
| id          | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
(...)
+-------------+------------------+------+-----+---------+----------------+

还有一个将在表t1，... tN中存储有关每个记录的注释的表：

 mysql> desc postit;

+---------------+------------------+------+-----+---------+----------------+
| Field         | Type             | Null | Key | Default | Extra          |
+---------------+------------------+------+-----+---------+----------------+
| id            | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| foreign_id    | int(10) unsigned | NO   | MUL | NULL    |                |
| foreign_table | varchar(20)      | NO   | MUL | NULL    |                |
| content       | text             | NO   |     | NULL    |                |
(....)
+---------------+------------------+------+-----+---------+----------------+

现在我应该如何为表't1'定义一个@Entity...。

@Entity(name="T1")
@Table(name="t1")
public class T1
    {
        (...)
        public List<PostIt> getComments() { ... }
        }

从表“ postit”检索其所有注释，因为表达式应为

select P from postit as P where P.foreign_table="t1" and P.foreign_id= :t1_id

是否有特定的JPA注释定义此关系，还是应该在T1的每个实例中注入（如何？） EntityManager并调用@NamedQuery？

Answer 1

我认为您应该使用继承对表进行建模。

这样就有了AbstractTable

@Entity
@DiscriminatorColumn(name="type_id")
@DiscriminatorValue("-1")
...
class AbstractTable {

@OneToMany(mappedBy="foreign_id")
public List<PostIt> getComments() { ... }
        }
...
}

并且：

@Entity
@DiscriminatorValue("1")
...
class Table1 {}

@Entity
@DiscriminatorValue("2")
...
class Table2 {}

如果需要双向关系，则需要弄清楚如何从Postit实体引用表。 我觉得像

@ManyToOne
public AbstractTable getTable() { ... }

应该管用。

可以在这里找到更多信息http://wleeper.com/2009/12/17/jpa-onetomany-with-inheritance/

Answer 2

您的设计与一棵继承树相对应，在该继承树中，使用Discriminator列（ foreign_table ）将根AbstractComment实体的所有子类存储在同一表中：

@Entity
@Table(name = "postit")
@Inheritance(strategy = SINGLE_TABLE)
@DiscriminatorColumn(name = "foreign_table", discriminatorType = STRING)
public abstract class AbstractComment {
    // ... fields, getters, setters
}

@Entity
@DiscriminatorValue("t1")
public class T1Comment extends AbstractComment {

}

@Entity
@Table(name="t1")
public class T1 {
    @OneToMany
    @JoinColumn(name = "foreign_id")
    private Set<T1Comment> comments;
}