JPA CriteriaBuilder - 按照一对多关系中关联实体的数量排序 [英] JPA CriteriaBuilder - sort by the number of associated entities in a one-to-many relationship

问题描述

我有两个实体Customer和Order，它们是一对多的关系。
对于每位客户，我需要统计相关订单的数量并按照该数字对结果进行排序。
在原生postgres查询中，它看起来像这样：

  select cust.id，count（order.id）from customers cust 
 left outer join order order 
 on cust.id = order.customer_id 
 where .... conditions ... 
 group by cust.id 
 order通过计数desc; 
  

但是我必须使用CriteriaBuilder来做这件事，因为这个查询是一个使用CriteriaBuilder的更大代码的一部分增加附加条件。在Hibernate中，我可能会使用Projections，但在JPA中找不到任何类似的东西。

使用CriteraBuilder编写查询的任何帮助都将非常感谢。

预先感谢您。

解决方案

假设实体Customer具有 OneToMany 属性如下：

  @OneToMany（mappedBy =customerId ）
私人收藏<订单>命令; 
  

您可以使用以下查询：

  EntityManager em; //被构建或注入
 CriteriaBuilder cb = em.getCriteriaBuilder（）; 
 CriteriaQuery< Tuple> cq = cb.createTupleQuery（）; 
根< Customer> customer = cq.from（Customer.class）; 
 CollectionJoin<客户，订单> orders = customer.join（Customer_.orders，JoinType.LEFT）; 
 cq.select（cb.tuple（customer，cb.count（orders）））; 
 cq.where（...在这里添加一些谓词...）; 
 cq.groupBy（customer.get（Customer_.id））; 
 cq.orderBy（cb.desc（cb.count（orders）））; 
 List< Tuple> result = em.createQuery（cq）.getResultList（）; 
 for（Tuple t：result）{
 Customer c =（Customer）t.get（0）; 
 Long cnt =（Long）t.get（1）; 
 System.out.println（Customer+ c.getName（）+has+ cnt +orders）; 
} 
  

上述方法使用元模型。如果你不喜欢它，你可以用orders和 Customer_.orders > Customer_.id 与id。

如果 OneToMany 属性是另一种类型，用适当类型（ ListJoin CollectionJoin $ c>， SetJoin ， MapJoin ）。

I have two entities Customer and Order in a one-to-many relation. For each customer I need to count the number of associated orders and sort the results by this number. In a native postgres query it looks like this:

select cust.id, count(order.id) from customers cust
left outer join orders order
on cust.id = order.customer_id
where .... conditions ...
group by cust.id
order by count desc;

But I must do this using CriteriaBuilder because this query is part of a larger piece of code that uses CriteriaBuilder to put in additional conditions. In Hibernate I would have probably used Projections, but I can't find anything similar in JPA.

Any help in composing the query using CriteraBuilder would be much appreciated.

Thank you in advance.

解决方案

Supposing that the entity Customer has a OneToMany property like this:

@OneToMany(mappedBy = "customerId")
private Collection<Order> orders;

You can use the following query:

EntityManager em;  // to be built or injected
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Tuple> cq = cb.createTupleQuery();
Root<Customer> customer = cq.from(Customer.class);
CollectionJoin<Customer, Order> orders = customer.join(Customer_.orders, JoinType.LEFT);
cq.select(cb.tuple(customer, cb.count(orders)));
cq.where(... add some predicates here ...);
cq.groupBy(customer.get(Customer_.id));
cq.orderBy(cb.desc(cb.count(orders)));
List<Tuple> result = em.createQuery(cq).getResultList();
for (Tuple t : result) {
    Customer c = (Customer) t.get(0);
    Long cnt = (Long) t.get(1);
    System.out.println("Customer " + c.getName() + " has " + cnt + " orders");
}

The above approach uses Metamodel. If you don't like it, you can replace Customer_.orders with "orders" and Customer_.id with "id".

If the OneToMany property is of another type, replace CollectionJoin with the collection of the proper type (ListJoin, SetJoin, MapJoin).

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