Hibernate (JPA) Entity with Static collection as a member

Question

Is it possible to have a static collection as a member of a hibernate entity?

Say I have an object Question:

public class Question {
    private String category;

    ...
}

Would it be possible to populate a static Set<String> that is a distinct set of all categories in the Database? I know I could just query this, but I was wondering if there was a more elegant solution, as it seems like something that other people may have come across.

Answer 1

To my knowledge, JPA cannot persist static or final fields. The spec isn't very clear on this but it makes IMO sense since I don't see what would be the one in a OneToXXX relation (so how should JPA load it). You might want to check the following thread for some feedback (i.e. not a final answer) from the ejb3 expert group. I'm afraid you'll have somehow to link all your Question with all Category.

But maybe you'll get more "satisfying" answers. Let's see.

Answer 2

Simple but not perfect solution would be using entity @PostLoad callback method on Category or Question entities (or both):

@Entity
public class Question {
  public static Set<String> categories = new HashSet<String>();
  ...

  @PostLoad
  public void fillInCategories() {
    categories.add(category);
  }

}

categories is lazy loaded - until all questions that contain every category are loaded it won't be complete.