Relationships are a missing feature of OO languages but an essential feature of real-world applications and data. Relationships between persistent objects are implemented using first-class link objects. The link objects manage:
The relationship framework provides key to object-based user interfaces. It automatically manages the following relationship semantics:
Figure 11. Relationships can be uni-directional and bi-directional. The diagram shows a two-way (bi-directional) relationship: a one-to-many and one-to-one relationship between the two classes.