JPA: please help understanding “join fetch”

Question

I have the following entity structure: Business --> Campaign --> Promotion, where ONE Business can have MANY Campaigns and ONE Campaign can have MANY Promotions. Both one-to-man

Answer 1

1. List can have redundant elements because of the cartesian product and people weren't aware of that, so the hibernate guys started throwing this error to force people to use Set instead of List to avoid this problem. Source 1