使用 entityManager.createNativeQuery(query,foo.class) [英] use of entityManager.createNativeQuery(query,foo.class)

问题描述

我想从一个

javax.persistence.EntityManager.createNativeQuery 调用

为什么以下不正确?

entityManager.createNativeQuery("Select P.AppID From P", Integer.class);

特别是为什么我得到...Unknown entity: java.lang.Integer"

specifically why do I get "...Unknown entity: java.lang.Integer"

我是否必须创建一个实体类，其中包含一个 Integer 字段?

Would I have to create an entity class that has a single field that is an Integer ?

谢谢

推荐答案

您所做的称为<​​strong>投影.那时您只返回属于一个实体的标量值.你可以用 JPA 做到这一点.请参阅标量值.

What you do is called a projection. That's when you return only a scalar value that belongs to one entity. You can do this with JPA. See scalar value.

我认为在这种情况下，完全省略实体类型是可能的:

I think in this case, omitting the entity type altogether is possible:

   Query query = em.createNativeQuery(  "select id from users where username = ?");  
   query.setParameter(1, "lt");  
   BigDecimal val = (BigDecimal) query.getSingleResult(); 

示例取自此处.

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