如何在 JPA 中加入 Criteria 多态？答案

【问题标题】：How do a Criteria polymorphic join in JPA?如何在 JPA 中加入 Criteria 多态？
【发布时间】：2013-10-23 16:13:18
【问题描述】：

我有一个项目有这样的建模：

public abstract class Actor{}
public class Person extends Actor {}
public class Organization extends Actor {}

public abstract class Role{ @ManyToOne @JoinColumn(name="ID_ACTOR") }
public class Customer extends Role{}
public class Employee extends Role{}

我想要一个List<Role>，它扮演一个特定的演员：

public List<Role> getRoles(Actor actor) {
  CriteriaBuilder builder = em.getCriteriaBuilder();
  CriteriaQuery<Role> criteria = builder.createQuery(Role.class);

  //How to do this filter using Criteria API (JPA 2.1)?

  return query.getResultList();
}

下面的 SQL 完成了这项工作：

SELECT 'Customer' as role, c.id as id, c.id_actor as actor
FROM customer c
left join person p on p.id = c.id_actor
left join organization o on o.id = c.id_actor
where c.id_actor = ?

UNION 

SELECT 'Employee' as role, e.id as id, e.id_actor as actor
FROM employee e
left join person p on p.id = e.id_actor
left join organization o on o.id = e.id_actor
where e.id_actor = ?

【问题讨论】：

你有没有处理过这个问题stackoverflow.com/questions/17212905/…。

标签： jpa jpa-2.0 criteria-api

【解决方案1】：

我解决了在 Actor 和 Role 类之间建立双向关系的问题：

public abstract class Actor{ @OneToMany(mappedBy="actor") List<Role> roles }
public class Person extends Actor {}
public class Organization extends Actor {}

public abstract class Role{ @ManyToOne @JoinColumn(name="ID_ACTOR") Actor actor }
public class Customer extends Role{}
public class Employee extends Role{}

这样我就可以获得扮演特定演员的List<role>：

public List<Role> getRoles(Actor actor) {
    return actor.getRoles();
}

最初我想避免双向关系，但我意识到在这种特殊情况下，有直接的好处

【讨论】：