An N-ary association is a relationship that exists between three or more classes. The association cannot be broken down into a regular, binary association without some information being lost. The great majority of associations are binary; very few are ternary (N = 3). Rarer still are associations of order four or more (N = 4 or more). A common example of a ternary association is the one that exists between a taxpayer, an accountant, and the IRS.
To draw an N-ary association
1 Connect two classes with an association line
2 Draw an association line from a third class to the association line between the first two classes.
The association line connects to the line, and a diamond-shaped symbol appears at the junction. The entire multi-part association becomes one structure, with one underlying definition.
3 To open the definition of the ternary association, double-clicking the line.
The definition dialog of the line contains an association end for each class involved in the association (so for a ternary association, there will be three association ends, and so on).
