如何在CriteriaQuery中查询超类的地方引用特定于子类的字段?

问题描述

我正在尝试使用JPA Criteria API实现以下目标:

I'm trying to achieve something like the following, using the JPA Criteria API:

SELECT b FROM Box b JOIN SpecialItem s WHERE s.specialAttr = :specialAttr

对象是

框

@Entity
public class Box implements Serializable {
  ...
  @ManyToOne
  @JoinColumn( name = "item_id" )
  Item item;
  ...
}

项目

@Entity
@Inheritance( strategy = InheritanceType.JOINED )
public class Item implements Serializable {
  @Id
  private String id;
  ...
}

SpecialItem

@Entity
public class SpecialItem extends Item {
  private String specialAttr;
  ...
}

我的尝试

EntityManager em = getEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery cq = cb.createQuery( Box.class );
Root from = cq.from( Box.class );

// Nothing to specify SpecialItem over Item!
Join join = from.join("item", JoinType.LEFT);

// java.lang.IllegalArgumentException: Unable to
// resolve attribute [specialAttr] against path [null]
Path p = join.get( "specialAttr" );

Predicate predicate = cb.equal( p, "specialValue" );
cq.where( predicate );

毫不奇怪，它会引发异常，因为specialAttr不是Item类的成员.

Not surprisingly it throws an exception because specialAttr isn't a member of class Item.

如何返回所有包含SpecialItem的Box，其中SpecialItem.specialAttr具有某些值?

How can I return all the Boxes that contain a SpecialItem, where the SpecialItem.specialAttr has some value?

推荐答案

如果使用JPA 2.1，则可以使用

If using JPA 2.1 you might use

"SELECT b FROM Box b WHERE TREAT(b.item as SpecialItem).specialAttr = :specialAttr"

或

CriteriaQuery<Box> q = cb.createQuery(Box.class);
Root<Box> box= q.from(Box.class);
Join<Box, Item > order = box.join("item");
q.where(cb.equal(cb.treat(order, SpecialItem.class).get("specialAttr"),
    qb.parameter(String.class, "specialAttr")));
q.select(Box);

这篇关于如何在CriteriaQuery中查询超类的地方引用特定于子类的字段?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！