When to use, not to use, OneToOne and ManyToOne

Question

I just started reading of JPA, and the implementation in hibernate to understand the details. but, to continue with development till then, can you help to clarify a basic qn

Answer 1

When to use OneToOne I may use OneToOne if the entity manager needs to handle the persistency of the related object. the point is, I can always live without specifying oneToOne, but then the responsibility is on me to manage the relationship and making sure that the referred objects are not in transient state. Is this true?

You need not mark the relationship if you don't want the entity manager to handle operations. You can have the related entity as a transient field and persist the related entity manually. But in that case, you wouldn't be using the whole list of features that JPA provides.

By marking the relationships, the EntityManager comes to know about the database structure. And that is essential if you want to utilize the power of JPA.

Eg: class Car
      {
         @OneToOne
         Warehouse warehouse;
         //other fields
      }

Here

• I can retrieve the related Warehouse object while retrieving a Car

• I can persist a Warehouse object while persisting a Car (with cascade option as explained by the poster below )

• likewise Update, Delete ...

Apart from these, while using JPQL,

you can traverse to Warehouse

as in

em.createQuery("select c.warehouse from Car c"); 

These wouldn't work unless you mark the relationship.