Definition:
Our first relationship is a simple association. In this relationship, one
first-rank class holds a reference to a single instance of another first-rank class
and they are related by a common PK. Scenario:
We have two first-rank classes, Foo and Bar which are related to each other as follows:
Bar Foo.getBar() // returns corresponding Bar instance
Hibernate Mapping:
In Hibernate, this could be mapped as follows:
No extra columns are needed to support this relationship. Instead both Foo and Bar must
share the same PK values to be part of a one-to-one assocation. If you create suitable instances of Foo and Bar with a shared PK, then retrieving a Foo will
automatically retrieve the corresponding Bar. Bidirectionality:
This relationship can be bidrectional, with Bar having getFoo(), by simply adding a similar
mapping and Foo property to Bar.