Spring Data JPA.如何仅从findAll()方法获取ID列表 [英] Spring Data JPA. How to get only a list of IDs from findAll() method

问题描述

我有一个非常复杂的模型.实体有很多关系，依此类推.

I have a very complicated model. Entity has a lot relationship and so on.

我尝试使用Spring Data JPA，并准备了一个存储库.

I try to use Spring Data JPA and I prepared a repository.

但是当我用对象的规范调用方法findAll()时，会出现性能问题，因为对象很大.我知道这是因为当我调用这样的方法时:

but when I invoke a method findAll() with specification for the object a have a performance issue because objects are very big. I know that because when I invoke a method like this:

@Query(value = "select id, name from Customer ")
List<Object[]> myFindCustomerIds();

我的表现没有任何问题.

I didn't have any problems with performance.

但是当我调用

List<Customer> findAll(); 

我对性能有很大的疑问.

I had a big problem with performance.

问题是我需要使用客户规格"来调用findAll方法，这就是为什么我不能使用返回对象数组列表的方法的原因.

The problem is that I need to invoke findAll method with Specifications for Customer that is why I cannot use method which returns a list of arrays of objects.

如何编写一种方法来查找所有具有客户实体规范但仅返回ID的客户.

How to write a method to finding all customers with specifications for Customer entity but which returns only an IDs.

像这样:

List<Long> findAll(Specification<Customer> spec);

• 在这种情况下，我无法使用分页.

请帮助.

推荐答案

我解决了这个问题.

(因此，我们将只有ID和名称为稀疏的Customer对象)

(As a result we will have a sparse Customer object only with id and name)

public interface SparseCustomerRepository {
    List<Customer> findAllWithNameOnly(Specification<Customer> spec);
}

和一个实现(记住后缀-默认为Impl)

@Service
public class SparseCustomerRepositoryImpl implements SparseCustomerRepository {
    private final EntityManager entityManager;

    @Autowired
    public SparseCustomerRepositoryImpl(EntityManager entityManager) {
        this.entityManager = entityManager;
    }

    @Override
    public List<Customer> findAllWithNameOnly(Specification<Customer> spec) {
        CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
        CriteriaQuery<Tuple> tupleQuery = criteriaBuilder.createTupleQuery();
        Root<Customer> root = tupleQuery.from(Customer.class);
        tupleQuery.multiselect(getSelection(root, Customer_.id),
                getSelection(root, Customer_.name));
        if (spec != null) {
            tupleQuery.where(spec.toPredicate(root, tupleQuery, criteriaBuilder));
        }

        List<Tuple> CustomerNames = entityManager.createQuery(tupleQuery).getResultList();
        return createEntitiesFromTuples(CustomerNames);
    }

    private Selection<?> getSelection(Root<Customer> root,
            SingularAttribute<Customer, ?> attribute) {
        return root.get(attribute).alias(attribute.getName());
    }

    private List<Customer> createEntitiesFromTuples(List<Tuple> CustomerNames) {
        List<Customer> customers = new ArrayList<>();
        for (Tuple customer : CustomerNames) {
            Customer c = new Customer();
            c.setId(customer.get(Customer_.id.getName(), Long.class));
            c.setName(customer.get(Customer_.name.getName(), String.class));
            c.add(customer);
        }
        return customers;
    }
}

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