Compared to the single-class apps discussed in the previous parts of this tutorial, we have to deal with a number of new technical issues:
In the model code we now have to take care of reference properties that require
defining referential integrity constraints with the help of suitable JPA annotations;
extending the code of the add, update and
destroy methods by taking the reference properties into consideration;
a method for converting between (internal) object references and corresponding (external) ID references in serialization and de-serialization operations.
In the user interface (view) code we now have to take care of
showing information about associated objects in the retrieve/list all use case;
allowing to select an associated object from a list of all existing instances of the target class in the create object and update object use cases.