Question

I was reading over the docs regarding Eclipselink’s support for @OrderColumn. It looks like this only applies to List and not Set. The reason I ask is because I have a ManyToMany bi-directional relationship (using a join table) which is a Set and is implemented with a HashSet because the collection can’t have duplicates.

I wanted to order the entries in this set using @OrderColumn, but it appears I can only apply this to List, however using List will break my unique requirement. Is this understanding correct?

If so what is the recommended strategy for this case?

Thanks,

-Noah

Answer 1

This looks similar to the following question:

Why cannot a JPA mapping attribute be a LinkedHashset?

The Set interface does not define ordering of elements, so your set needs to be a concrete implementation like a TreeSet or LinkedHashSet implementation, not just any old Set. But your JPA provider is generally going to use its own collection implementations with special magic to handle lazy loading.

The above answer suggests that there may be some EclipseLink-specific workaround if you are willing to give up lazy loading.

I can think of two options, neither one perfect:

• just use a List and rely on business logic to enforce uniqueness, with DB UNIQUE constraints as a backstop. Honestly, I end up using List for collections almost reflexively, even when Set would have been more appropriate; I admit it’s sloppy but has yet to cause any significant problems for me in years of practice.

• use a Set and change @ManyToMany to @OneToMany, and make your join table w/order column an actual entity that implements Comparable using the order column. Then, overload your getter method to do something like

  if (! this.set instanceof TreeSet) 
     this.set = new TreeSet<T>(this.set); 
  return this.set;