JPQL: How to "SELECT new Foo(null, null... someValue, ..)?

Question

For example I have an entity ``` public class Foo { private String col1; private String col2; private String col3; private String col4; //getters and setters } ``` What I want to do is to `select` only `col3` and `col4`. But I already have a `Foo` constructor like below: ``` public Foo (String col1, String col2) { this.col1 = col1; this.col2 = col2; } ``` Thus, I cannot have another constructor for `col3` and `col4` because it will have the same signature. What I am trying to accomplish so far is to make a complete constructor like: ``` public Foo (String col1, String col2, String col3, String col4) { this.col1 = col1; this.col2 = col2; this.col3 = col3; this.col4 = col4; } ``` But when I try to do something like below in my query ``` SELECT new Foo(null, null, f.col3, f.col4) FROM Foo f ``` I get `org.hibernate.hql.internal.ast.QuerySyntaxException: unexpected end of subtree` Although when I try ``` SELECT new Foo(f.col1, f.col2, f.col3, f.col4) FROM Foo f ``` It works as expected. E…

Accepted Answer

You can try (but just a try)

```
SELECT new Foo(cast(null as string), cast(null as string), f.col3, f.col4)
FROM Foo f
```

or looking about cast() operator for JPQL if it is supported (I'm not sure) else

```
Session session = entityManager.unwrap(Session.class);
List<Foo> list = session.createQuery("select f.col3, f.col4 from Foo f").list()
```

This is Hibernate specific and you don't need specific constructor for every projections; just create an empty constructor `Foo()` (at least protected, I don't remember if private is allowed) and let Hibernate inject value into `col3, col4` from your query (and `cast()` operator is supported if your underlying database support the function).

JPQL: How to "SELECT new Foo(null, null... someValue, ..)?

Solution

Problem