Spring Data JPA. How to get only a list of IDs from findAll() method - spring

Question

I have a very complicated model. Entity has a lot relationship and so on.
I try to use Spring Data JPA and I prepared a repository.
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();
I didn't have any problems with performance.
But when I invoke
List<Customer> findAll();
I had a big problem with performance.
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.
How to write a method to finding all customers with specifications for Customer entity but which returns only an IDs.
like this:
List<Long> findAll(Specification<Customer> spec);
I cannot use in this case pagination.
Please help.

Answer 1

Why not using the #Query annotation?
#Query("select p.id from #{#entityName} p")
List<Long> getAllIds();
The only disadvantage I see is when the attribute id changes, but since this is a very common name and unlikely to change (id = primary key), this should be ok.

Answer 2

This is now supported by Spring Data using Projections:
interface SparseCustomer {
String getId();
String getName();
}
Than in your Customer repository
List<SparseCustomer> findAll(Specification<Customer> spec);
EDIT:
As noted by Radouane ROUFID Projections with Specifications currently doesn't work beacuse of bug.
But you can use specification-with-projection library which workarounds this Spring Data Jpa deficiency.

Answer 3

I solved the problem.
(As a result we will have a sparse Customer object only with id and name)
Define their own repository:
public interface SparseCustomerRepository {
List<Customer> findAllWithNameOnly(Specification<Customer> spec);
}
And an implementation (remember about suffix - Impl as default)
#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;
}
}

Answer 4

Unfortunately Projections does not work with specifications. JpaSpecificationExecutor return only a List typed with the aggregated root managed by the repository ( List<T> findAll(Specification<T> var1); )
An actual workaround is to use Tuple. Example :
#Override
public <D> D findOne(Projections<DOMAIN> projections, Specification<DOMAIN> specification, SingleTupleMapper<D> tupleMapper) {
Tuple tuple = this.getTupleQuery(projections, specification).getSingleResult();
return tupleMapper.map(tuple);
}
#Override
public <D extends Dto<ID>> List<D> findAll(Projections<DOMAIN> projections, Specification<DOMAIN> specification, TupleMapper<D> tupleMapper) {
List<Tuple> tupleList = this.getTupleQuery(projections, specification).getResultList();
return tupleMapper.map(tupleList);
}
private TypedQuery<Tuple> getTupleQuery(Projections<DOMAIN> projections, Specification<DOMAIN> specification) {
CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Tuple> query = cb.createTupleQuery();
Root<DOMAIN> root = query.from((Class<DOMAIN>) domainClass);
query.multiselect(projections.project(root));
query.where(specification.toPredicate(root, query, cb));
return entityManager.createQuery(query);
}
where Projections is a functional interface for root projection.
#FunctionalInterface
public interface Projections<D> {
List<Selection<?>> project(Root<D> root);
}
SingleTupleMapper and TupleMapper are used to map the TupleQuery result to the Object you want to return.
#FunctionalInterface
public interface SingleTupleMapper<D> {
D map(Tuple tuple);
}
#FunctionalInterface
public interface TupleMapper<D> {
List<D> map(List<Tuple> tuples);
}
Example of use :
Projections<User> userProjections = (root) -> Arrays.asList(
root.get(User_.uid).alias(User_.uid.getName()),
root.get(User_.active).alias(User_.active.getName()),
root.get(User_.userProvider).alias(User_.userProvider.getName()),
root.join(User_.profile).get(Profile_.firstName).alias(Profile_.firstName.getName()),
root.join(User_.profile).get(Profile_.lastName).alias(Profile_.lastName.getName()),
root.join(User_.profile).get(Profile_.picture).alias(Profile_.picture.getName()),
root.join(User_.profile).get(Profile_.gender).alias(Profile_.gender.getName())
);
Specification<User> userSpecification = UserSpecifications.withUid(userUid);
SingleTupleMapper<BasicUserDto> singleMapper = tuple -> {
BasicUserDto basicUserDto = new BasicUserDto();
basicUserDto.setUid(tuple.get(User_.uid.getName(), String.class));
basicUserDto.setActive(tuple.get(User_.active.getName(), Boolean.class));
basicUserDto.setUserProvider(tuple.get(User_.userProvider.getName(), UserProvider.class));
basicUserDto.setFirstName(tuple.get(Profile_.firstName.getName(), String.class));
basicUserDto.setLastName(tuple.get(Profile_.lastName.getName(), String.class));
basicUserDto.setPicture(tuple.get(Profile_.picture.getName(), String.class));
basicUserDto.setGender(tuple.get(Profile_.gender.getName(), Gender.class));
return basicUserDto;
};
BasicUserDto basicUser = findOne(userProjections, userSpecification, singleMapper);
I hope it helps.