如何使用Hibernate Projection检索复杂的类及其成员？ [英] How to retrieve a complex class and its members using Hibernate Projection?

问题描述

我有一个类，如下所示，需要使用Hibernate从数据库中检索。
问题是我的课有多个成员，其中大部分都是类，我该如何找回它们？

  @实体
 public class Student {
 @Id 
 long id; 
字符串名称; 
字符串fname; 
 @OneToMany 
列表<课程>培训班; 
 @ManyToOne 
经销商; 
 ... 
} 
 
 @实体
公共类经销商{
 @Id 
 long id; 
字符串名称; 
 @OneToMany（fetch = FetchType.LAZY，mappedBy =cr.dealer，cascade = CascadeType.ALL）
 Set< Car> cars = new HashSet< Cars>（0）; 
 .. 
 
} 
  

我需要检索学生ID 1和它的所有课程，其经销商和经销商的汽车列表。

我的预测如下，但它不返回任何内容。

  ... 
 .setProjection（Projections.projectionList（）
 
 .add（Projections.property（friends.cars ）.as（cars）
 ... 
  

解决方案因为你有一个课程列表和一组汽车，你可以简单地在一个查询中获取整个图表：

 从学生s中选择s 
 
离开连接获取s.courses 
离开连接获取s.dealer d 
离开连接获取d.cars 
其中s .id =：id 
  

因为您正在提取两个集合，所以此查询将生成一个笛卡尔积，您需要确保所选的儿童收藏品没有太多入场券。

如果您不想要要运行到笛卡尔产品，您可以简单地运行此查询：

 从学生s $ b $中选择s 
 b left join fetch s.courses 
 left join fetch s.dealer d 
 where s.id =：id 
  

然后您访问dealer.cars以使用单独的查询获取该集合：

 学生s = ...; 
 s.getDealer（）。getCars（）。size（）; 
  

I have a class as following that need to retrieve from DB using Hibernate. The problem is my class has multiple members and majority of them are classes, how can I retrieve them?

@Entity
public class Student {
  @Id
  long id;
  String name;
  String fname;
  @OneToMany
  List<Course> courses;
  @ManyToOne
  Dealer dealer;
  ...
}

@Entity
public class Dealer {
   @Id
   long id;
   String name; 
   @OneToMany(fetch = FetchType.LAZY, mappedBy = "cr.dealer", cascade = CascadeType.ALL)
   Set<Car> cars = new HashSet<Cars>(0);
   ..

}

I need to retrieve student id 1 and all its courses, its dealer and list of dealers' cars.

My projection is as following but it does not return anything.

  ...
    .setProjection(Projections.projectionList()

    .add(Projections.property("friends.cars").as("cars")
    ...

解决方案

Because you have a List of Courses and a Set of Cars, you can simply fetch the whole graph in a single query:

select s
from Student s
left join fetch s.courses
left join fetch s.dealer d
left join fetch d.cars
where s.id = :id

Because you are fetching two collections, this query will generate a Cartesian Product, so you need to make sure that the selected children collections don't have too many entries.

If you don;t want to run into a Cartesian product, you can simply run this query:

select s
from Student s
left join fetch s.courses
left join fetch s.dealer d
where s.id = :id

and then you access the dealer.cars to fetch that collection with a separate query:

Student s = ...;
s.getDealer().getCars().size();

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