Object Relational Mappers

Java: Hibernate

• If you specify a one-to-many or many-to-many mapping, be sure to use the same mapping as the Java collection interface. A Set mapping won't work with a List collection, for example.

• In Hibernate, lazy loading is on by default for collections. This means the collections are fetched from the database only when they are used (never if they are not used). Warning: if the loading appears when the Hibernate session is already closed, it will fail.

• If you have an object and a many-to-one mapping on this object (such as Item that belongs to a Category), and you happen to have the ID of the Category but not the associated Java object, in order to store the item on the database the fastest way is to create a new virtual Category and call setID(known_id) on this newly created object. Then, once you save the object, the virtual Category will not be saved (updated) as it already exists.