DDD Anti-Pattern #4: Allowing Implementation Decisions to Drive the Domain Model

This is the last (for now) in my series of lessons learned building a complex product from the ground up following the principles of Domain Driven Design.

Appointment to WorkOrder

The Field Service domain is all about getting people to Locations to perform Activities. The Activities and the Location are defined by a Work Order. The person performing the Activities (and possibly the time) are defined by an Appointment. You can think of the Work Order as the what and where and the Appointment as the who and when.

Many Appointments to WorkOrder

The model is simple as long as there is one Work Order and one Appointment. In fact, you’d be tempted to combine them into one entity. However, things get more complicated when you have one Work Order that requires multiple Appointments to complete it. It might be two technicians at one time or the same technician on two different visits or multiple technicians over multiple visits.

Activities to Appointment to WorkOrder

In these cases, we are splitting the Activities of the Work Order over multiple Appointments. Some of the Activities are associated with one Appointment and some with another.

The Work Order is complete when all Activities over all Appointments are complete. Not too complicated.

Now also imagine that a technician goes out on one Appointment but services more than Work Order.Activities to Appointment to WorkOrder 2

In simplest terms, we have a many-to-many between Appointments and Work Orders. You can imagine the twisted case where a given Appointment services two different Work Orders, and each of those Work Orders has other Appointments serving them.

We’ve all implemented many-to-many relationships in databases and in object models. However, this situation is a little different because of the role of the Activity. The linkage between the Appointment and the Work Order is through their association with a common Activity.

Appointment to WorkOrder through ActivityA given Activity is associated with one Work Order and one Appointment. We can now traverse that relationship to discover the relationship between Appointment and Work Order.

We simply have Work Orders with collections of Activities and Appointments with collections of Activities. With that, the complex relationship between Appointment and Work Order is completely modeled, with no redundant connections (like something directly linking Appointments and Work Orders). I thought this was pretty cool. And it was.

But here is where the problem comes in… In the real world, Appointments really are associated with Work Orders—and not just by the happenstance of common Activities. (One could argue this actually is true because you would only schedule an Appointment to service a Work Order if there was some Activity to perform, but this is not how anyone thinks about the relationship.)

With our model, to answer the question “Where does this Appointment take place?” we have to go to one of the Activities in our collection, navigate to its Work Order, and look at its Location property. This not a big deal to do in code, and we can even put a property on the Appointment that hides this messiness and gives the illusion that an Appointment has a Location. But this is not the only difficulty.

Try looking in our database to see what Work Order a given Appointment is associated with. You have to go to the Activity table and find all the Activities associated with this Work Order, then look at the Work Order column of those Activities to find the ID(s) of the Work Order(s). One day I tried to create a view that shows an Appointment and its first Work Order (there was only ever one Work Oder per Appointment in practice). I gave up after 20 minutes.

Now imagine explaining this model to a programmer trying to integrate with our system. Its possible, but there is much more confusion than you’d like.

We had a technically tight and elegant implementation, but it ended up obscuring one of the most fundamental relationships in our domain.


As much as we liked the normalization of our current implementation, the team decided that it would be better to model the relationship between Appointment and Work Order explicitly. If we had focused more on the domain and less on the cleverness of the implementation, we could have avoided this rework of the design.

SOLID: What is Old is New Again


Sometime around 2001, I came across a series of articles by Robert C. Martin. At that time they were already a little old—from The C++ Report in 1996. I didn’t really care. This guy (who I now know as everyone else does as “Uncle Bob”) had captured and expressed core principles that I knew from experience were the key to creating maintainable software. I mostly knew it mostly from having violated them and paying the price.

The principles put forth in those articles express fundamentals truths that are in play in any Object-Oriented software project.

In the fifth article in the series, Martin sums up the first four…

1. The Open Closed Principle. (OPC) January, 1996. This article discussed the notion that a software module that is designed to be reusable, maintainable and robust must be extensible without requiring change. Such modules can be created in C++ by using abstract classes. The algorithms embedded in those classes make use of pure virtual functions and can therefore be extended by deriving concrete classes that implement those pure virtual function in different ways. The net result is a set of functions written in abstract classes that can be reused in different detailed contexts and are not affected by changes to those contexts.

2. The Liskov Substitution Principle. (LSP) March, 1996. Sometimes known as “Design by Contract”. This principle describes a system of constraints for the use of public inheritance in C++. The principle says that any function which uses a base class must not be confused when a derived class is substituted for the base class. This article showed how difficult this principle is to conform to, and described some of the subtle traps that the software designer can get into that affect reusability and maintainability.

3. The Dependency Inversion Principle. (DIP) May, 1996. This principle describes the overall structure of a well designed object-oriented application. The principle states that the modules that implement high level policy should not depend upon the modules that implement low level details. Rather both high level policy and low level details should depend upon abstractions. When this principle is adhered to, both the high level policy modules, and the low level detail modules will be reusable and maintainable.

4. The Interface Segregation Principle. (ISP) Aug, 1996. This principle deals with the disadvantages of “fat” interfaces. Classes that have “fat” interfaces are classes whose interfaces are not cohesive. In other words, the interfaces of the class can be broken up into groups of member functions. Each group serves a different set of clients. Thus some clients use one group of member functions, and other clients use the other groups.

The ISP acknowledges that there are objects that require non-cohesive interfaces; however it suggests that clients should not know about them as a single class. Instead, clients should know about abstract base classes that have cohesive interfaces; and which are multiply inherited into the concrete class that describes the non-cohesive object.

Martin later published The Single Responsibility Principle, which says that there should never be more than one reason for a class to change.

There were several aspects of these short articles that helped me so much…

  • The articles were short but clear.
  • The articles were available on-line for anyone to download and read.
  • Martin gave these principles names.

All of these aspects made these principles easy to promote—and I did! I preached all of these principles, whether in C++ or later C#. So, I am particularly pleased to see the recent resurgence in these principles as the ALT.NET crowd has arranged them into a nice acronym (SOLID) and made back-to-basics cool again. We see the SOLID principles all over blogs and articles in the agile community. It reaffirms my belief that the fundamentals may go in and out of style, but they are always what matters most in good software development.