java - JPA : defining a complex @OneToMany relationship

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 |
(...)
+-------------+------------------+------+-----+---------+----------------+

以及一个表，该表将存储有关表 t1, ... tN 中每条记录的一些注释:

 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 @Entity ....

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

从表“postit”中检索其所有评论，如表达式所示

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

是否有特定的 JPA 注释来定义此关系，或者我应该在 T1 的每个实例中注入(inject)(如何？) EntityManager 并调用 @NamedQuery ？

Answer 1

我认为你应该使用 inheritance 为表建模.

这样你就有了 AbstractTable

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

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

还有:

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

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

如果您需要双向关系，那么您需要弄清楚如何从 Postit 实体引用表。我认为类似

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

应该可以。

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