How to cast a referenced entity in HQL to its sub-class

Question

Suppose there are following entities (JPA annotations are not written):

class Questionnaire {
    ...
}

class Policy {
    private Questionnaire questionnaire;
    ...
}

class LifeQuestionnaire extends Questionnaire {
    private String someField;
}

class LifePolicy extends Policy {  
    ...
}

Well as it seems a Policy references a Questionnaire, but a LifePolicy references a LifeQuestionnaire (this restriction is always true).

Is there any way I can write a query like this:

from LifePolicy lplc
where ((LifeQuestionnaire) lplc.questionnaire).someField = :fieldValue

As you can see, I want to have some kind of casting in the HQL query, because someField is only available in LifeQuestionnaire.

Answer 1

I had exactly the same problem and have found a solution that worked for me. Its not very nice but it works. Try using a 'Cross' join. e.g.

select (specify what you want to fetch) 
from LifePolicy as lp, LifeQuestionnaire as lqn 
inner join lqn.questionnaire as q 
where lqn.id = lp.id 
and q.someField = :fielfValue

sources:

https://www.jumpingbean.co.za/blogs/mark/hibernate https://docs.jboss.org/hibernate/orm/3.3/reference/en/html/queryhql.html#queryhql-from