JPQL: Receiving a Collection in a Constructor Expression

Question

I\'m using JPQL and want to receive some normal parameters and a collection in a Constructor Expression to directly create the DTO-objects. But if the Collection is empty, I

Answer 1

Try,

public DTO (long id, String name, Object children)

Answer 2

This is not an answer.

The JPA 2.0 spec doesn't, as far as i can see, allow the use of collections as parameters in constructor expressions. Section 4.8 defines a constructor expression like this:

constructor_expression ::=
        NEW constructor_name ( constructor_item {, constructor_item}* )
constructor_item ::=
        single_valued_path_expression |
        scalar_expression |
        aggregate_expression |
        identification_variable

A single_valued_path_expression is what it sounds like - a property expression that points to a scalar of some sort (such as p.id), a scalar_expression is also an expression that points to scalar, an aggregate_expression is the application of a function like sum which reduces a multi-valued expression to a scalar one, and an identification_variable is a reference to the type you're querying over. None of which can be collection-valued. Assuming i've read the spec right.

So, if your JPA provider lets you use collection-valued parameter in a constructor expression, it's because it's going above and beyond the spec. Which is very nice of it, but not something you can necessarily rely on!

Answer 3

I had a similar problem and tried "Object" in DTO constructor as James suggested, but a child object is passed in and it seems that it is only the first child instead of the expected List/Array of children.

I ended up with a "normal" query creating all the DTOs in a for loop.

TypedQuery<Parent> query = em.createQuery("SELECT p FROM Parent p", Parent class);
        List<Parent> list = query.getResultList();
        List<DTO> result = new ArrayList<>();
        for (Parent p : list)
        {
            DTO dto = new DTO();
            //set dto props and fill collection
            result.add(obj);
        }
        return result;