Question

Let’s depict the following use case: I have a JPQL Query which on the fly creates data objects using the new keyword. In the SELECT clause I would like to inject an attribute which is not known to the database but to the layer which queries it.

This could look like

EntityManager em; // Got it from somewhere
boolean editable = false; // Value might change, e.g. depending on current date

Query q = em.createQuery("SELECT new foo.bar.MyDTO(o, :editable) FROM MyObject o")
            .setParameter("editable", editable);

List<MyDTO> results = (List<MyDTO>) q.getResultList();

Any ideas how this kind of attribute or parameter injection into the SELECT clause might work in JPQL? Both JPA and JPA 2.0 solutions are applicable.

Edit: Performance does not play a key role, but clarity and cleanness of code.

Answer 1

It will not work without possible vendor extensions, because according specification:

4.6.4 Input Parameters
…
Input parameters can only be used in the
WHERE clause or HAVING clause of a query.