Note: This is going to sound crazy at first, but bear with me.

The current best-practice for writing rails code dictates that your business logic belongs in your model objects. Before that, it wasn't uncommon to see business logic scattered all over controller actions and even view code. Pushing business logic in to models makes apps easier to understand and test.

I used this technique rather successfully for quite some time. With plugins like resource_controller, building an app became simply a matter of implementing the views and the persistence layer. With so little in the controller, the focus was mainly on unit tests, which are easier to write and maintain than their functional counterparts. But it wasn't all roses.

As applications grew, test suites would get slow — like minutes slow. When you're depending on your persistence objects to do all of the work, your unit tests absolutely must hit the database, and hitting the database is slow. It's a given in the rails world: big app == slow tests.

But slow tests are bad. Developers are less likely to run them. And when they do, it takes forever, which often turns in to checking twitter, reading reddit, or a coffee break, harming productivity.

Also, coupling all of your business logic to your persistence objects can have weird side-effects. In our application, when something is created, an after_create callback generates an entry in the logs, which are used to produce the activity feed. What if I want to create an object without logging — say, in the console? I can't. Saving and logging are married forever and for all eternity.

When we deploy new features to production, we roll them out selectively. To achieve this, both versions of the code have to co-exist in the application. At some level, there's a conditional that sends the user down one code path or the other. Since both versions of the code typically use the same tables in the database, the persistence objects have to be flexible enough to work in either situation.

If calling #save triggers version 1 of the business logic, then you're basically out of luck. The idea of creating a database record is inseparable from all the actions that come before and after it.

Here Comes the Crazy Part

The solution is actually pretty simple. A simplified explanation of the problem is that we violated the Single Responsibility Principle. So, we're going to use standard object oriented techniques to separate the concerns of our model logic.

Let's look at the first example I mentioned: logging the creation of a user. Here's the tightly coupled version:

class User < ActiveRecord::Base after_create : log_creation protected def log_creation Log . new_user ( self ) end end

To decouple the logging from the creation of the database record, we're going to use something called a service object. A service object is typically used to coordinate two or more objects; usually, the service object doesn't have any logic of its own (simplified definition). We're also going to use Dependency Injection so that we can mock everything out and make our tests awesomely fast (seconds not minutes). The implementation is simple:

class UserCreationService def initialize ( user_klass = User , log_klass = Log ) @ user_klass = user_klass @ log_klass = log_klass end def create ( params ) @ user_klass . create (params). tap do | u | @ log_klass . new_user (u) end end end

The specs:

describe UserCreationService do before do @ user = stub ( " User " ) @ user_klass = stub ( " Class:User " , : create => @ user ) @ log_klass = stub ( " Class:Log " , : new_user => nil ) @ service = UserCreationService . new ( @ user_klass , @ log_klass ) @ params = { : name => " Matz " , : hobby => " Being Nice " } @ service . create ( @ params ) end it " creates the user with the supplied parameters " do @ user_klass . should have_received ( : create ). with ( @ params ) end it " logs the creation of the user " do @ log_klass . should have_received ( : new_user ). with ( @ user ) end end

Aside from being able to create a user record in the console without triggering a log item, there are a few other advantages to this approach. The specs will run at lightning speed because no work is actually being done. We know that Fast specs make happier and more productive programmers.

Also, debugging the actions that occur after save becomes much simpler with this approach. Have you ever been in a situation where a model wouldn't save because a callback was mistakenly returning nil? Debugging (necessarily) opaque callback mechanisms is hard.

But then I'll have all these extra classes in my app!

Yeah, it's true. You might write a few more "class X; end"s with this approach. You might even write a few percent more lines of actual code. But you'll wind up with more maintainability for it (not to mention faster tests, code that's easier to understand, etc).

The truth is that in a simple application, obese persistence objects might never hurt. It's when things get a little more complicated than CRUD operations that these things start to pile up and become pain points. That's why so many rails plugins seem to get you 80% of the way there, like immediately, but then wind up taking forever to get that extra 20%.

Ever wondered why it seems impossible to write a really good state machine plugin — or why file uploads always seem to hurt eventually, even with something like paperclip? It's because these things don't belong coupled to persistence. The kinds of functionality that are typically jammed in to active record callbacks simply do not belong there.

Something like a file upload handler belongs in its own object (at least one!). An object that is properly encapsulated and thus isolated from the other things happening around it. A file upload handler shouldn't have to worry about how the name of the file gets stored to the database, let alone where it is in the persistence lifecycle and what that means. Are we in a transaction? Is it before or after save? Can we safely raise an error?

In the tightly coupled version of the example above, the interaction between the User object and the Log object are implicit. They're unstated side-effects of their respective implementations. In the UserCreationService version, they are completely explicit, stated nicely for any reader of our code to see. If we wanted to log conditionally (say, if the User object is valid), a plain old if statement would communicate our intent far better than simply returning false in a callback.

These kinds of interactions are hard enough to get right as it is. Properly separating concerns and responsibilities is a tried, tested, and true method for simplifying software development and maintenance. I'm not just pulling this stuff out of my ass.