Today, we will learn how the Query Method pattern handles a set of tensions we often face while designed object oriented systems.
What are patterns? Patterns are learnable and reusable answers to common programming questions. They are decisions made in response to a number of different concerns (or tensions). A well written pattern resolves each of the tensions harmoniously and provides a set of repeatable steps for its reapplication.
Let’s look at a situation where various tensions pull us towards the need to make a harmonious design decision and attempt to derive a pattern for future use. As we do so, we will try to keep in mind the tensions at play and come up with steps that can be used to reapply the pattern in the future.
The Query Method pattern answers a particularly common question, “How can I ask an object for information about itself?” The tensions at play include a desire to maintain proper encapsulation, a desire to keep code DRY(Don’t Repeat Yourself) and a desire to maintain or improve the readability of the resulting solution.
If you don’t really care about the discovery process and you want to skip straight to the pattern itself, it’s at the bottom.
Discovering A Pattern
In a recent blog post on the value of learning encapsulation before learning rails, Stephen Chu gives an excellent example of a fragment of ruby code that could be improved by application of the Query Method pattern. I’ll use a slightly different example:
Let’s look at this code in light of some of the different tensions:
In his post, Stephen points out that this code breaks encapsulation by yanking state information out of the object for comparison. The comparison is being made on post information but it is being made outside the context of the post. We can change the code a bit to make this more clear:
The blog that is creating this recent post list shouldn’t and needn’t know how to determine if a post is recent. This is the post’s responsibility. The blog should simply ask the post if it is recent and let the post make the calculation. This desire for encapsulation is the first tension.
Don’t Repeat Yourself
This code seems simple enough. It does one thing and appears to do it efficiently. There is no violation of DRY yet, but it isn’t to difficult to imagine other situations where we might need to know if a post is recent.
Let’s say we are generating an HTML page and we want to apply a special class to recent posts. We will need to duplicate this logic inside that method as well. What if we later decide that we want posts in the last three days instead of two? Now we have to find each instance of this code fragment so that we can change it appropriately.
What a pain! Clearly this code is not DRY and its maintainability is compromised as a result. This is the second tension.
This code looks pretty readable.
2.days.ago reveal their
intention quite clearly. If any improvement is to be made, you may think, it
will be incremental and relatively small. Nevertheless, improved readability
means improved maintainability, so it is always worth consideration. This is
the third tension.
Composing A Pattern
This list of tensions isn’t exhaustive but it should be sufficient for us to begin considering a solution. How can we improve this code in a way that most effectively addresses all of them?
This is a case where the tensions are not in direct opposition. It should be pretty easy to come up with a solution. Let’s try to formulate such a pattern, including the steps necessary to reapply it again in the future.
Encapsulation and DRY
First, we’ll start by encapsulating the comparison inside the post object:
This resolves the encapsulation tension. By factoring the behavior to a single location it also resolves the DRY tension. It doesn’t do much for our readability tension, though. Let’s see what we can do about that.
Patterns can often be improved by the inclusion of smaller, more granular patterns. In this case, the readability can be improved by providing the method with an Intention Revealing Selector. An intention revealing selector is, as the name suggests, one that informs to the user of the intention of the method rather than its implementation.
Our current selector - and the code it is based on - informs the user of the method’s implementation. It answers the “how” question. An Intention Revealing Selector answers the more useful “why” question, as in: why does this method exist, why would I want to use it?
So: why does this method exist? It exists so that the post can be asked if it is recent or not. An intention revealing name for this selector would use the word “recent” to answer the “why” question.
The Ruby idiom for a method that is intended to return a boolean value is to end it with a “?”. Let’s apply the Intention Revealing Selector pattern by giving the method an intention revealing name and ending it with a ”?”.
I think you’ll agree that this new method resolves all of the tensions harmoniously. The behavior is properly encapsulated, it is DRY because it is defined Once And Only Once, and it is significantly more readable than the original because it reveals its intention rather than its implementation.
Thus, we can formulate the Query Method Pattern as such:
Formulating The Query Method Pattern
When you want to query an object about itself in a way that is properly encapsulated, DRY and more readable, follow these steps:
- Write a method that performs the query.
- Put it inside the object that holds the information being queried.
- Give it an Intention Revealing Selector.
The next time you’re faced with this design question, you can apply the Query Method pattern.
Hopefully this process will help you begin to identify patterns of your own. If you’re interested in learning more about patterns (and you should be), I highly recommend Smalltalk Best Practice Patterns by Kent Beck (non-affiliate link). It’s worth learning Smalltalk just to be able to understand the code examples.