Today we continue looking at query expressions, which I introduced with much hullabaloo last time, but have thus far only given you the smallest taste. The glutinous among you are surely clamoring for more. So let's eat. But please stop using that use case diagram as a napkin. (Javadoc is much more absorbent).
Sarge has for the first time invited us to the headquarters of Arms4Less.com. After our blindfolds have been removed and our hands untied, we find ourselves seated around a large conference table in a windowless room. Charts around the wall show sales projections and troop strength estimates for the various markets Arms4Less is targeting over the next twelve months.
Last time we looked at how we can derive a value (isTrusted) using a query expression. Let's consider another such example. Sarge explains to us that Arms4Less prides itself on its high ethical standards. "Like if we sell a particular model of tank to one guy, for a small surcharge of 10%, we won't sell the same kinda tank to any of his enemies." We consider this requirement and realize that we need to prevent a customer from ordering any product that has been previously ordered by any of the customer's enemies. Now many of you are probably wonder how the heck we can express something like this on a lowly class diagram. Doesn't this require a more procedurally-oriented specification? Something with a little more get-up-and-go than a class diagram? Class diagrams just lay there, you say. We need something that moves, don't we?
Ha! I laugh at you (with all due respect of course). Do you think I would have wasted this much time blogging about this crap if we could go no further? Consider our latest version of the precision model, below:
We have added a new association, enemies, to keep track of a customer's enemies, and we have added a derived association called enemyProducts that references all the products purchased by a customer's enemies. Let me clarify a few things about "derived associations". First, you may be scratching your head about why we're calling enemyProducts an association at all, since it's not represented by a line on our class diagram. Notice however the type of the enemyProducts attribute. It's a Product. (In fact, as we'll discuss another time, it's a list of Products.)
There are multiple ways of representing associations in UML. For example, the following two representations are virtually equivalent.
On top, we see that the Employee has a BattleInjury reference called injuries. We see exactly the same thing with the Employee representation on the bottom, which contains a BattleInjury reference as well. The only difference is the notation being used. On the top we depict the reference with a composite association named injuries, whereas on the bottom, we decpict the reference with a nested attribute, also called injuries.
A derived association then is simply a derived attribute whose formula evaluates to a list of entities of some type. Let's look at the derivation expression for enemyProducts:
In evaluating the value of this expression for a particular customer, we navigate through the customer's associated enemies, to their orders and then to the order's associated items. We then subset (query) the orders to select out those for which enemies have opted for an exclusive deal, and then we navigate from this resulting set to their associated products. The resulting list of products are the ones that we can't sell to the Customer.
The astute reader (or at least most who weren't out drinking last night) will have noticed we're essentially navigating multiple paths in parallel when we navigate enemies.orders.items since a customer can have many enemies, each in turn having many orders, and each of those orders having many items. The expression "unions" the results of navigating these parallel paths.
Now, you may recall earlier in this tirade we talked about the importance of precision. In fact, recall that all these techniques we're using comprise a methodology called Precision Modeling. You can rest assured that I'm not just whipping out arbitrary syntax for each of the formulas we've used. All of them are expressed with a formal expression language that are part of what we call Gorilla UML (GUML).
GUML not only has formal syntax definition. It executes! (Sarge says, "Cool! I execute too!") More on execution later. In our next little visit together, we'll actually look at using GUML to specify constraints. Using constraints we can complete our specification of Sarge's business rule preventing a product being sold to a customer's enemies.