Sep 1, 2017 1:00:27 PM | Top Tips for Refactoring Fat Models in Rails

A collection of tips and tricks to help you with refactoring rails models to ensure they live up to the single responsibility principle.

For most Rails developers, a massive benefit of using the Rails framework is the power of its core components, like the dominant Active Record. Acting as the model in the core model-view-controller paradigm that Rails is built around, according to the official documentation, an Active Record is the "layer of the system responsible for representing business data and logic. Active Record facilitates the creation and use of business objects whose data requires persistent storage to a database." In other words, an Active Record model should represent a data layer component (typically a database table), and each instance of a particular model is a one-to-one representation of an object in the database. This relationship is typically known as Object Relational Mapping (or ORM).

As Uncle Ben (Spider-Man's very own kin) is oft-quoted, "With great power comes great responsibility." This is certainly applicable in the nerdy realm of Rails' models, because it's all too easy to write code with the assumption that the notion of all "business data and logic", for which models are intended, encompasses anything and everything that doesn't directly relate to the view or controller components of the MVC architecture. For many developers, particularly those new to the Rails platform, projects can quickly spiral out of control with rapidly growing models, taking on more and more responsibility as the application scope increases. Without explicit care and precise refactoring, you may quickly find you've got one or more "fat models" (as they're colloquially known) on your hands. Like tiny little bulls made up of ones and zeroes, these monstrosities can wreak havoc in the china shop that is your codebase.

In our 5 Common Mistakes in Rails Development article, we looked at a handful of issues that frequently trip up both new and seasoned Rails developers. One topic we briefly discussed is obese models, promising a follow-up article taking a deeper dive into this issue and how to manage it. In today's article we'll tackle just that problem, examining the nature of fat models, the goals you should strive for in your own model design, and a few tips and tricks to help you with refactoring rails models that have already grown a little to big for their britches. Let's get this party started!

What Makes a Model Fat?

There are many indicators that your models have grown too big, but the most indicative is when your model breaks the single responsibility principle. First introduced by software engineer and author Robert Cecil Martin in his book Agile Software Development, Principles, Patterns, and Practices, the single responsibility principle states that every class in an application should have responsibility over one, and only one aspect of the overall software. Martin defines "responsibility" as a "reason to change," indicating that each class should only ever have one reason to change.

For example, imagine a class that handles sending out email confirmations to users after sign up. This class might include just a few fields like email and content. Perhaps, when initially creating this class, it was decided that each user has a unique email address, so the email field (which is a string) is sufficient. However, later down the road, there's eventually a need to associate this class with more complex relationships, because, perhaps users can register through some identifier besides their email address.

Now we're in a situation where this simple class intended to send out email confirmations actually has more than one responsibility or reason to change. One possible reason is when emails need to be sent out in different ways, perhaps based on their frequency, recipients, or email protocol type. Another reason to change is when we want to move from the old email identification to a more robust means, such as an association with a User class object found elsewhere in the application. This will allow users to be contacted not just by email, but by text message or otherwise.

Anyway, we could get into a deep rabbit hole of theoretical examples, but the same principles apply: If a model has grown to the point that some part of it is handling multiple responsibilities, it's best to try to refactor it and trim it down so it focuses solely on its single, independent job.

Extract Value Objects

A value object is an immutable class that is compared using its value rather than its identity. Common examples of value objects include numbers, dates, monetary values, and strings. For example, a penny is a value object. I can have five unique copies of a penny in my pocket at once, but their usefulness as currency is not dependant on their unique identification. Instead, they are identified based on their value as a representation of one cent. Similarly, the string "ABC" is equal to a second copy of the string "ABC". Even though these are two unique objects, their equivalence is always based on their value, rather than their identify.

As it happens, looking for value objects within your model is a great way to refactor and slim things down. For example, consider an application with a Book class. We allow users to rate a Book from 1 to 5, and we keep track of the total number of ratings in an array. We can then retrieve the current #rating by averaging all the values thus far:

class Book
def initialize(title, author, ratings)
@author = author
@ratings = ratings
@title = title

def add_rating(rating)

def rating
# Average ratings.
@ratings.inject{:+}.to_f / @ratings.size

While this method works, its rather limiting. What if we want a more complex form of rating? What if we need to perform additional logic based on the ratings that were provided? We can accomplish these goals, while also slimming down our primary Book class, by extracting the rating-related methods and placing them into their own value object class, Rating:

class Rating
def initialize(value)
@value = value || 1

def <=>(other)
other.to_s <=> to_s

def eql?(other)
to_s == other.to_s

def hash

def to_s

Now the Book class can reference the Rating class, rather than just a plain integer value, and we can perform more advanced logic within Rating as needed.

Define Service Objects

Another useful type of class to create, particularly as part of model refactoring, is a service object. A service object is a class that encompasses complex behaviors. Service objects often make use of multiple models at once, lending themselves to the notion that no single model should "own" this particular logic or behavior.

For example, consider an infrequent maintenance activity like cleaning up the Book records in the database every quarter. Perhaps the system should look at all Books and determine if their Ratings are below a certain threshold. If not, such Books should no longer be featured elsewhere in the application. While this code could be placed inside the Book model, it wouldn't make logical sense because the Book class represents a single instance of a book, rather than the whole collection of books. Therefore, this is a perfect scenario in which to use a service object class:

class BookCleanup
def initialize

# Remove featured flag from all books with rating below 3.
def cleanup
Book.where('rating < ?', 3).update_all(featured: false)

As a simple example, here the BookCleanup service object is used to change the featured flag to false for all Books with Ratings below 3.

Create Form Objects

Somewhat similar to a service object, a form object is ideal when handling a form request that manipulates multiple models at once. For example, when a User submits a rating on a book, this (may) require manipulation of the Book model and the User model. While we could choose to handle this within either the Book or the User model, doing either would break the single responsibility principle we're aiming for, so creating a separate form object class to handle this update is ideal:

class RatingForm
extend ActiveModel::Naming
include ActiveModel::Conversion
include ActiveModel::Validations

attribute :user, Integer
attribute :book, Integer
attribute :rating, Integer

validates :book,
presence: true
validates :rating,
presence: true,
numericality: {
only_integer: true,
greater_than_or_equal_to: 1,
less_than_or_equal_to: 5

def persisted?

def save
if valid?


def persist!
rating =
user = User.find(:user)
book = Book.find(:book)
book.add_rating!(rating, user)
user.add_rating!(rating, book)

There are many more options available to trim down extraordinarily fat models, but hopefully this brief look at a few critical tips and ways to slim down these models can help you in your own future Rails projects.

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Written By: Frances Banks