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On logic in a Rails app

published on 24 Dec 2012, 3 minute read

Recently, Grant Ammons wrote a nice article about where logic should lie in a Rails app. I would like to complement his opinion, and probably add some real life arguments to it.

Grant extracted logic from the controller, and put it in a separate object, which I also do most of the time. However, a user named DHH (which probably is DHH) said:

So all you’ve done here is extract the work of the controller action into a separate object. This adds needless indirection, makes the code base harder to follow, and is completely unnecessary.

I strongly disagree, and I have good reasons for it.

You know what makes the code base hard to follow? Fat controllers with logic in them. I worked on a Rails app with Controllers of 800 lines of code, it. is. a. freaking. nightmare.

My approach

I look at Rails as web presentation layer, it makes my app work in the browser. This has some consequences to my codebase:

You might think that this is a refactor pattern, and you are right. And you might say that this unnecessary to do from the start, and you may also be right. But the cost of creating a file and moving behaviour there is exactly 0, but the benefits are there from day 1.

Why I do it

Fast tests

The least I am coupled to Rails, the fastest my tests are. Currently, on a medium Rails project my tests run 2-3 seconds, where 90% of the time is wasted to boot rails and test models.

I don’t test my controllers, because they have no logic in them. My methods in controllers don’t have more than 4-5 lines of code.

class RatingsController < ApplicationController
  def create
    success = RatesBusiness.add_rating(params[:business_id],
    render json: {success: success}

RatesBusiness has some foursquare-related logic besides just creating a entry in the database. Using this approach, I have a very fast TDD flow, because most of the time I don’t need Rails to test my logic. (any dependency to models is stubbed, as any other external dependency).

Your controller only connects your logic to the web interface.

Code and forget

If you have a well tested functionality that works, you don’t need see it in front of you when you code in your application. This is where single-responsibility objects kick in, I TDD them one time, I know about their existence and what their interface is, I will not ever see their code again.

Framework agnostic

When your app logic is not coupled with Rails, you do not depend on it, its version, or interface. Isolating some functionality to a service is a matter of moving some files around.

Reduced stress and debug times

Most of my classes fit in one screen, and have descriptive names as CategorizesCampagins or FiltersBusinesses, which makes debugging them very, very easy and pleasant. I don’t need to scroll the hell out of my screen to understand how an isolated piece of functionality works.


Rails is just a framework to make your app work on the web, don’t mix your logic with rails-y stuff. Your app domain should be defined anywhere besides Rails views, controllers, and models.


First, and most influential person for me is Gary Bernhardt and his screencast series destroyallsoftware. If you want to learn isolated testing - watch this guy, he is amazing. (He even test-drives bash scripts :D)

Your objects the UNIX way.

Also, my approach was influenced by Hexagonal Rails concept.

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