我正在尝试构建一个较小的SQL,以避免默认情况下为hibernate Criteria构建的“select * from A”.

如果我使用简单的字段(无关系),通过“变形金刚”,我可以设法拥有这个SQL：

select description, weight from Dog;

嗨,我有这个实体：

@Entity
public class Dog
{
   Long id;
   String description;
   Double weight;
   @ManyToOne(fetch = FetchType.LAZY)
   @JoinColumn(name = "person_id", nullable = false)
   Person owner;
}

@Entity
public class Person
{
   Long id;
   String name;
   Double height;
   Date birthDate;
}

我的目标是拥有：

select description, weight, owner.name from Dog

我用Criteria(和子标准)尝试了这个：

Criteria dogCriteria = sess.createCriteria(Dog.class);
ProjectionList proList = Projections.projectionList();
proList.add(Projections.property("description"), description);
proList.add(Projections.property("weight"), weigth);
dogCriteria.setProjection(proList);

Criteria personCriteria = dogCriteria.createCriteria("owner");
ProjectionList ownerProList = Projections.projectionList();
ownerProList.add(Projections.property("name"), description);    
dogCriteria.setProjection(ownerProList);  //After this line,  debugger shows that the
                                          //projection on dogCriteria gets overriden
                                          //and the query fails, because "name" is
                                          //not a field of Dog entity.

我应该如何使用Projections,以获得更小的SQL,更少的列？
提前致谢.

解决方法:

首先,

select description, weight, owner.name from Dog

是无效的SQL.它必须是这样的

select description, weight, Person.name
 from Dog join Person on Dog.person_id = Person.id

代替.其次,为什么？虽然可以做你想做的事情(见下文),但是通过Criteria API这样做非常冗长,你无需为此付出任何代价.除非所述列是巨大的blob或者您选择了数十万条记录,否则几列的数据传输节省可以忽略不计.在任何一种情况下,都有更好的方法来处理这个问题.

任何人,为了做你想要的标准,你需要通过别名加入链接表(Person)并使用所述别名指定主要标准的投影：

Criteria criteria = session.createCriteria(Dog.class, "dog")
 .createAlias("owner", "own")
 .setProjection( Projections.projectionList()
   .add(Projections.property("dog.description"))
   .add(Projections.property("dog.weight"))
   .add(Projections.property("own.name"))
 );

在Criteria Projections documentation中有上述描述和示例.请记住,执行时,上述条件将返回对象数组列表.您需要指定ResultTransformer才能将结果转换为实际对象.