A set is a collection that has no sense of order, and a particular object can only appear in a set once. Sets are primarily useful for asking the question “Is it in there?” For example, you might have a set of URLs that are not child-appropriate. Before displaying any web page to a child, you would want to do a quick check to see if the URL is in the set. Sets do this incredibly quickly.

An employee’s assets have no inherent order, and an asset should never appear twice in the same employee’s assets collection. Change your program to use an NSMutableSet instead of an NSMutableArray for the assets relationship.

Figure 21.2  Using NSMutableSet for assets

In Employee.h, change the variable’s declaration: