Associations

Relationships between entities

Types

Reflexive association

Same object appears under different roles Arity (N-ary associations)
Number of linked entities, where >2 is rare and must be distinguished
Constraints include (optional linking, mandatory linking, uniqueness, etc…) Object specialization/generalization
Specialization = sub-classing (extending a class for specificity, the parent class generalizes the child class)
Inheritance is a consequence here
Can have a discriminator which is an attributes whose values define different specialization Decompositon/aggregation
An object can belong weakly to several containers or strongly to one
To distinguish, ask can an object exist without its container?
Apply composition to aggregates of physical objects or when parts share a lifecycle with the whole Ordered associations
Denote order OR associations
Same role can be played by multiple objects Associations of associations
One of the linked objects is an association

We can represent associations through a tuple of linked objects kind of like a Dependancy Array

A unique association used at one end to distinguish uniquely identify something on the other end of the association (like a PK)