Encapsulation: Entities, Collections And Business Rules
Yesterday, I was involved two very separate yet very related conversations. One was via twitter with Colin Jack and Jimmy Bogard (which I was only a partial contributor to – mostly just reading their conversation) and another after work with a coworker. The short version of both conversations can be boiled down to encapsulation of logic surrounding collections that are held by entities. Rather than rehash all of the conversations, I wanted to specifically address a violation of encapsulation that I’ve seen many times when dealing with collections and business rules.
Let’s look at a small example where we have a medical system that deals with Patients that are having Consultations with doctors. There is a need to keep track of the historic consultations and also the current consultation, if any. For simplicity we’ll say that the current consultation is identified as being the most recent, based on a starting date, and that any previous consultation must have an ending date. A very simplistic implementation of an object model to represent patients and consultations may look something like this:
Then, when the time comes to add a new consultation to the patient, making it the current one and closing a previous consultation, code may get called from somewhere in the application (like a code behind of a form, or if you’re lucky, in the Presenter or Controller of an MVP/C setup), like this:
From a purely technical standpoint, there is nothing wrong with this code. It implements the business rule as defined. However, this code misses out on some great opportunities to encapsulate the rules we have into a process that can be called from anywhere that the system needs it – whether or not the code is in the specific presenter / controller that creates a new consultation or not. The very same code that comprises this implementation could easily be placed in the Patient object, abstracting the rules and process of creating a new consultation into a simple, single method call.
Before I show how I would approach that solution, though, there is one other implementation that I’ve seen recently that not only breaks encapsulation, but has to compensate for the lack of rules enforcement with logic in the wrong place. Instead of storing the current consultation as a set value, the CurrentConsultation property may have some logic in it to dynamically determine which consultation is the current one.
This type of code is a huge encapsulation violation smell. Since our Patient object has no enforcement of the consultations that it holds, there is no way for us to really know which consultation is the current one. Because of this, the retrieval of the current consultation has to process the entire consultation collection and try to find the most recent consultation that has no ending date.
On top of the encapsulation issue, we have lost a great deal of performance. We now have to loop through the list every time we need the current consultation. If the list is small, this might not be such a bad problem, but as the list grows and as this code is used more and more, the performance problem may have a serious impact on the system.
Fortunately, the solution to the encapsulation violation, the enforcement of the business rules and the performance problem can all be wrapped up in to some very simple code. The first thing we want to do is prevent the ad-hoc addition of consultations to the patient. We still need to access the list of consultations, but we don’t really have a need to modify it outside of the patient class itself. This can be done with a one-line code change to the Patient class’s Consultations property:
Now that we have prevented the ability to do ad-hoc consultation additions, we need a way to actually add consultations. While we are doing this, we also want to enforce the business rules of the current consultation as described earlier. This is where we are going to take much of the original code that we found in the presenter / controller and place it into the patient class directly.
In the end, this code shows a better encapsulation of the business rules and logic that surrounds the need to maintain a list of consultations and a current consultation. With this code in place, we could simplify the presenter / controller that we talked about initially. Rather than being forced to know all of that logic in the presenter, we can make one simple method call:
With this one simple call, we have a guaranteed execution of the business rules that we need. This will allow us to recreate the ability to add a new consultation at any point in the application that we need, not just in the original presenter / controller that we were working with.
Part of the conversation via twitter revolved around where this type of logic should be encapsulated. From what I gathered, Colin tends to place this logic in custom collection objects, which would allow him to call patient.Consultations.Add() and still encapsulate the same business rules into that method. Like everything else in software development, there are multiple ways to solve the same problem. What does your specific situation, project, team, and business context need? Whatever your implementation needs are, though, we need to keep this type of logic and business rules enforcement well encapsulated in our systems.