Using Service Objects in Ruby on Rails

Once an application reaches certain size, questions about architecture begin to appear. Rails follows a Model View Controller organization and basic rules exist for clean code:

  • No Fat Models – don’t allow them to get bloated
  • Keep views dumb – don’t put any logic in there
  • Keep controllers skinny – don’t put too much there

And it raises the first question: Where do I put all that code?

Introducing service objects

Service objects can be a class or a module in Ruby that perform an action and can help take out logic from other areas of the MVC structure. For a simple example, let’s say we have a controller like this:

class PostsController < ApplicationController
  def create
    @title = params[:title]
    @content = params[:content]
    @post = Post.new(title: @title, content: @content)
    if @post.save
      flash.notice = 'Post saved'
      render @post
    else
      flash.alert = flash_error_message(@post)
      redirect_to :new
    end
  end
end

Extracting some of this into a service object is easy once you understand the design pattern.

  • create a services folder in the Rails’ app folder
  • create the service object file, in this example create_post.rb
  • extract the functionality to the CreatePost class/module
  • reload the Rails app and try it

Service objects as modules

Using a module approach I created a service that looks very much like a factory design pattern:

module CreatePost
  class << self
    def execute(params)
      title = params[:title]
      content = params[:content]
      post = Post.new(title: title, content: content)
    end
  end
end

Which in turn made the controller a lot more manageable:

class PostsController < ApplicationController
  def create
    @post = CreatePost.execute(params)
    if @post.save
      flash.notice = 'Post saved'
      render @post
    else
      flash.alert = flash_error_message(@post)
      redirect_to :new
    end
  end
end

Service objects as classes

In some cases we need to store instance variables and other methods, if so, we use classes. Using a class, our code could be rewritten as:

class CreatePost
  def initialize(params)
    @title = params[:title]
    @content = params[:content]
  end

  def call
    Post.new(title: @title, content: @content)
  end
end

The code of the controller would be:

class PostsController < ApplicationController
  def create
    @post = CreatePost.new(params).call
    if @post.save
      flash.notice = 'Post saved'
      render @post
    else
      flash.alert = flash_error_message(@post)
      redirect_to :new
    end
  end
end

Organizing service objects with modules

When we start using services our services folder tends to grow a lot. We can manage this growth by creating a modular structure using folders and modules.

The services folder can reflect the variety of service objects and it’s different uses in our app. We group them in namespaces using Ruby modules.

module Post
  module Build
    def self.call(params)
      title = params[:title]
      content = params[:content]
      Post.new(title: title, content: content)
    end
  end
end

To achieve this we have to place them in folders that reflects our module structure to let Rails load them.

services/post/build.rb
services/post/update.rb
services/comments/build.rb
...

This way our use of service objects can scale with the growth of our app.

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