Java JPA: Select 'ATTRIBUTE is in List<>'

Question

I am using JPA and trying to select all childentities (Product) of a given List of parents (Category). The relationship is Category OneToMany Product. I would like to keep it down to one query and not create a Predicate like product.get("category") == category.get(0) || product.get("category") == category.get(1) || ....

I have tried the following bit of code, but this does not seem to work (as seen in the stack at the bottom). Does anyone have a suggestion as how to accomplish this?

Code

public List<Product> findProductsBy(List<Category> categories) {
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<Product> query = cb.createQuery(Product.class);
    Root product = query.from(Product.class);
    Predicate predicateCategory = product.get("category").in(categories);

    query.select(product).where(predicateCategory);
    return em.createQuery(query).getResultList();
}

Stack

WARNING: Local Exception Stack: 
Exception [EclipseLink-6075] (Eclipse Persistence Services - 2.3.2.v20111125-r10461): org.eclipse.persistence.exceptions.QueryException
Exception Description: Object comparisons can only use the equal() or notEqual() operators.  Other comparisons must be done through query keys or direct attribute level comparisons. 
Expression: [
Relation operator [ IN ]
   Query Key category
      Base (...).Product

Answer 1

What you can do is using the category id instead of the category object (as the error state: object comparison is not supported):

(Assuming your category id is a Long, you can do the following.)

public List<Product> findProductsBy(List<Category> categories) {
    List<Long> categoryIds = new ArrayList<Long>();
    for(Category category:categories){
        categoryIds.add(category.getId());
    }
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<Product> query = cb.createQuery(Product.class);
    Root product = query.from(Product.class);
    Predicate predicateCategory = product.get("categoryId").in(categoryIds);

    query.select(product).where(predicateCategory);
    return em.createQuery(query).getResultList();

}