Concept: Association

2. Definition

An association describes a relationship between two classes of object that must be remembered by a system. For example, in a complaint management system an incident triggers a complaint. The system must remember which complaints were triggered by which incidents. Associations can also encapsulate the basic business rules of an organisation. For example an insurance claim must be associated with an insured person.

The vast majority of associations involve two classes (binary) but on occasion can involve three (ternary) or more. Where possible all associations should be decomposed into binaries as ternaries and above are difficult to conceptualize.

3. Describing a Binary Association

In a binary association is fully described by indicating:

• Each class' relationship with the other
• Its cardinality and
• Its optionality.

In the complaint management example the association between incident and complaint is fully described by:

• an incident triggers a complaint
• a complaint is made about an incident

Note that the association names are read from the model in a clockwise direction.

Cardinality

Cardinality is expressed in terms of:

• one to one
• one to many
• many to many

Optionality is expressed in terms of:

• Must have. For example, a purchase order must have a line item. In other words, it is mandatory.
• May have. For example, a purchase order may have a requisition number. In other words, it is optional.

Two classes may have more than one relationship. For example:

4. Describing a Ternary Association

A ternary association describes a fact that involves three classes and cannot be split up into component binary associations without losing information. For example, when a seller negotiates a price with a buyer through an agent, a decomposition of the ternary into two binary associations between buyer and seller and seller and agent would lose the information on which buyer was involved in the deal between the seller and agent.

A ternary association is described in terms of the role that each class plays in the association. For example:

• An agent negotiates a price between a buyer and a seller
• A buyer negotiates a price with a seller through an agent
• A seller negotiates a price with a buyer through an agent

5. Identifying Associative Classes

Associative classes are used when it is useful to keep information about the association itself. For example when a customer purchases a book, information on the actual purchase does not belong with the customer or the book, it is a class in itself. We therefore create the class purchase.

Associative classes are common in many to many associations.

6. Using Roles in Associations

Role names describe the role played in an association by a class. Using role names adds clarity to the model. For example, when a person manages other persons in a team it is useful to indicate the two roles played in the association by the same class. In the case of a team leader/team member association a person who plays the role of team leader manages zero, one or many persons who play the role of team member.

7. Using Aggregates

Aggregates provide a shorthand method of modeling associations between objects when they are the components of an assembly.

Recursive Aggregates

A recursive aggregate contains an instance of the same kind of aggregate. For example, a computer program is an aggregation of blocks with optionally recursive compound statements; the recursion terminates with simple statements. Blocks can be nested to arbitrary depth.

8. Example

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Module: 104 v1.0 c_assocn.htm
Updated: July 02, 2006