Question

In JPA, I am trying to create an Update screen for an JPA object with a list of categories for that object. This is a many to many relationship.

I am creating a join relationship on the table within the JPA object that has the following properties.

Classes.java

@JoinTable(name = "classes_has_class_categories",joinColumns = {
@JoinColumn(name = "classes_id", referencedColumnName = "id")}, inverseJoinColumns = {
@JoinColumn(name = "class_categories_id", referencedColumnName = "id")})
@ManyToMany(cascade = CascadeType.ALL)
private Collection<ClassCategories> classCategoriesCollection;

The key to my question is contained within the below logic as I have updated the database and everything is great within the DB. However, my problem is that when I access the object at a later time, the classCategoriesCollection property within the object doesn’t have all of the correct objects. I would have thought that my flush and refresh calls below would have synchronized everything up with the database, but that object property is still empty UNTIL I redeploy/recompile the application. Please help me with knowing how to get this object to sync up with the current database in the object.

ClassManager.java

for(String c : cats)
{
      int cId = ConvertToInt(c);
      ClassCategories ca = em.find(ClassCategories.class, cId);

      if(ca != null)
      {

        try{
           if(!cla.getClassCategoriesCollection().contains(ca))
           {
               ClassHasCategoriesPK classHasCategoriesPK= new ClassHasCategoriesPK();

               classHasCategoriesPK.setCategoryId(ca.getId());
               classHasCategoriesPK.setClassId(cla.getId());

                ClassHasCategories cd = new ClassHasCategories(classHasCategoriesPK);

                em.persist(cd);
                em.flush();                               

            }
        }
        catch (Exception ex)
        {}    
        }
      }

      em.persist(cla); //cla is the Class object instance.
      em.flush();
      em.refresh(cla);

Later on in the code….

request.setAttribute("classCategories",cl.getClassCategoriesCollection());

Doesn’t have the right classes until a recompile of the application.

Answer 1

Why do you have a ClassHasCategories class for a M_N relationship table when your relationship has no extra parameters? Since Classes has a @ManyToMany(cascade = CascadeType.ALL) relationship with ClassCategories, all you need to be doing is something like:

classObj.getClassCategoriesCollection().add(classCategoryObj);
em.merge(classObj).

The provider will take care of updating the relationship table. You can also remove items from classCategoriesCollection using similar logic:

classObj.getClassCategoriesCollection().remove(classCategoryObj);
em.merge(classObj).

So, the answer is:

• If classes_has_class_categories has no columns but the pk with two fks, get ride of ClassHasCategories and ClassHasCategoriesPK. Work with classCategoriesCollection to update the relationship table.
• If classes_has_class_categories has extra columns, then redo your mapping in a way that ClassHasCategoriesPK has two @ManyToOne relationships, respectively for Classes and ClassCategories. Update relationships by directly manipulating ClassHasCategories. Here is a code example.

The rule of thumb is, never mix both strategies (like you are doing by having both a @ManyToMany relationship and a separate ClassHasCategories object for the same table).

Cheers,