JPA查询仅选择特定列而不使用Criteria Query？ [英] JPA Query selecting only specific columns without using Criteria Query?

问题描述

是否可以在不使用条件查询的情况下使用JPA查询从对象中选择属性A和B？

Is it possible to select, say, only properties A and B from an object using a JPA query without using criteria queries?

要选择所有属性我会只需执行以下操作：

To select all properties I'd just do something like:

SELECT i FROM ObjectName i WHERE i.id = 10

但我在遗留系统上有一个很多属性的对象，即使我想要选择几个我知道选择几个属性通常很快。

But I have an object with many properties on a legacy system, and want to select just a few even though I'm aware selecting several properties is usually quick.

这是否可以不使用条件查询？

Is this possible without using criteria queries?

谢谢！

推荐答案

是的，就像在普通的SQL中一样，您可以指定要选择的属性类型：

Yes, like in plain sql you could specify what kind of properties you want to select:

SELECT i.firstProperty, i.secondProperty FROM ObjectName i WHERE i.id=10

执行此查询将返回Object []列表，其中每个数组包含一个对象的选定属性。

Executing this query will return a list of Object[], where each array contains the selected properties of one object.

另一种方式是将所选属性包装在自定义对象中并在TypedQuery中执行：

Another way is to wrap the selected properties in a custom object and execute it in a TypedQuery:

String query = "SELECT NEW CustomObject(i.firstProperty, i.secondProperty) FROM ObjectName i WHERE i.id=10";
TypedQuery<CustomObject> typedQuery = em.createQuery(query , CustomObject.class);
List<CustomObject> results = typedQuery.getResultList();

例子可以在这篇文章。

更新29.03.2018：

UPDATE 29.03.2018:

@Krish：

@PatrickLeitermann对我来说它给出了引起：org.hibernate.hql.internal。 ast.QuerySyntaxException：无法找到类***异常。如何解决这个问题？

@PatrickLeitermann for me its giving "Caused by: org.hibernate.hql.internal.ast.QuerySyntaxException: Unable to locate class ***" exception . how to solve this ?

我猜你在Spring应用程序的上下文中使用JPA，不是吗？其他一些人正好同样的问题他们的解决方案是在 SELECT NEW 关键字之后添加完全限定名称（例如com.example.CustomObject）。

I guess you’re using JPA in the context of a Spring application, don't you? Some other people had exactly the same problem and their solution was adding the fully qualified name (e. g. com.example.CustomObject) after the SELECT NEW keywords.

也许内部实现Spring数据框架只识别使用 @Entity 注释的类，或者通过简单名称在特定的orm文件中注册，这会导致使用此解决方法。

Maybe the internal implementation of the Spring data framework only recognizes classes annotated with @Entity or registered in a specific orm file by their simple name, which causes using this workaround.

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