Moe, Larry, and Curly Braces

We've thus far discussed several of the primary pillars of Precision Modeling -- derivation expressions, conditional expressions, query expressions, and derived associations -- and we've used these to help out our hypothetical (and some might say homicidal) customer, arms4less.com ("where you can get regime change for small change!"). We've been specifying the business rule stating that customers' can pay a surcharge to prevent their enemies from purchasing any of the same weapons they're purchasing themselves. We did this by creating a derived association, enemyProducts, that contains the list of all products we are unable to sell to a particular customer, because they have already been purchased by enemies of the customer who have opted to pay the surcharge:

/enemyProducts : Product = enemies.orders.items[isExclusive].product

Now we just need to somehow use this association to prevent a customer from actually buying any of these enemyProducts. As with the many wonders of Precision Modeling we've previously discussed, we can specify such a requirement precisely and concisely (Hey, that rhymes. Our marketing people will be so pleased.) via a little Gorilla UML.

Notice the constraint expression we've attached to OrderItem:

{not product in order.customer.enemyProducts}

In UML, constraints are specified inside curly braces (many have postulated this proves once and for all that Grady Booch is a die-hard Three Stooges fan). Because this constraint is attached to OrderItem, it is specifying that any product associated with an OrderItem cannot also be included in the list of the order's customer's enemyProduct's list.

Our SME, Sarge, inquires as to how we plan to enforce this constraint? "If it gets violated, there needs to be -- you know -- consquences." We ensure him that we will be sure to thoroughly test our implementation of this constraint to ensure that the software indeed enforces it. Sarge sighs wistully as he removes his hand from a bulge beneath his flack jacket.