Rob Davarnia

Ruby on Rails

Fat models, skinny controllers? How about skinny models, and skinny controllers?

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Fat models, skinny controllers? How about skinny models, and skinny controllers?


MVC (Model-View-Controller) design pattern provides a great architecture for our applications. We all have heard “Fat models, Skinny Controllers” but is that really true? Well, the answer is not quite simple.

Web Applications are changing

Web apps have evolved so much in the past few years. It’s now so common to use third-party API, real time notifications, queueing systems, and etc. But, at heart all applications require their own business rules.

Why not Fat models, Skinny Controllers?

It’s a common thought that taking logic out of controllers and dumping it in models will make the controller skinny. Technically it will make the controller skinny, but the model gets cluttered and messy! So, what should we do?

Skinny Models, Skinny Controllers, and Service Objects

The goal is to avoid making one object do everything (aka. The God Object). Instead, dividing logic in smaller methods that hopefully do one thing. Not only those methods are easier to test, but also very easy to maintain. Service objects can help take care of a Model’s non-database driven methods and application logic. Service objects can also reduce code duplication because other objects can use its methods.


  • Separation of concerns is your friend!
  • Don’t be afraid of making classes and objects! They can help simplify your code.
  • Use service objects for application logic
  • Avoid Controller and Model bloat
  • Don’t Repeat Yourself (DRY)


Further reading:

If you are in the Ruby world, I highly recommend checking out Practical Object-Oriented Design in Ruby (POODR), by Sandi Metz.



Rob Davarnia

I’m a Developer from Orange County, California. I'm a huge supporter of Open Source Projects, and making Web a better place.

  • user

    AUTHOR Dustin

    Posted on 10:32 pm February 24, 2015.

    I recommend watching / reading Robert C. Martin’s “Clean Code” series – especially the more recent videos. I think you will enjoy and embrace them.

    • user

      AUTHOR Rob Davarnia

      Posted on 8:22 pm June 14, 2015.

      Thanks for the recommendation Dustin! I own a copy of Clean Code, haven’t watched the recent videos.

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