JPA CriteriaBuilder - sort by the number of associated entities in a one-to-many relationship

问题

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.

回答1:

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).

来源：https://stackoverflow.com/questions/14425674/jpa-criteriabuilder-sort-by-the-number-of-associated-entities-in-a-one-to-many