Spring Data JPA Query - class projection containing another custom object

Question

Suppose I have a Foo entity (fields: id , name and barId ) and a Bar entity (fields: id , age ), in a one-to-many relation. I want to select all the Foo s, having the corresponding Bar age attached.

I see 2 options, but none seems to work:

1. The so-called closed projection approach

@Query("select f as foo, b.age as age from Foo f inner join Bar b on f.barId = b.id")
List<FooWithAge> query1();

where FooWithAge is an interface having as methods Foo getFoo() and Integer getAge() ;

2. The so-called class projection approach

@Query("select new FooWithAge(new Foo(f), b.age) from Foo f inner join Bar b on f.barId = b.id")
List<FooWithAge> query2();

where FooWithAge is a class having as fields Foo foo and Integer age and a constructor that takes these 2 as parameters.

None of these solutions seem to work. A third solution, of taking all the fields of Foo as parameters of the constructor of FooWithAge works, but I would like to find a cleaner solution.

Note: I cannot use anything else than vanilla Spring Data JPA queries. Native queries are excluded.

Answer 1

1. For foo object you have to define a sub-interface:

public interface FooWithAge {
 
    String getAge();
    FooView getFoo();
     
    interface FooView {
        //list of getters of foo
    }
}

2. In the first parameter you are creating a foo in a foo, so basically you have just to pass f :

@Query("select new FooWithAge(f, b.age) from Foo f inner join Bar b on f.barId = b.id")
List<FooWithAge> query2();