When you're working on a Ruby on Rails application, the Model-View-Controller (MVC) pattern usually captures most of what you want the app does. But not everything fits neatly into MVC. Sometimes, you need to make decisions about where to put logic that doesn't quite belong in a model, view, or controller. How you organize that logic determines whether your code will be easy for the next developer to understand—or whether they’ll be cursing your name.

Choosing the right abstraction for non-MVC logic is not just about keeping your code clean; it’s about making sure the next person who touches your code can pick it up, understand it quickly, and make changes confidently. This is how we build maintainable Rails applications. Let’s take a look at the patterns that help keep your Rails app simple, structured, and intuitive for others: Services, Concerns, Form Objects, Policy Objects, Query Objects, Interactors, and Engines.

Example:

Services: Clear Responsibilities Make Code Easy to Follow

Service Objects are the perfect place to encapsulate business logic. When you see services in a Rails app, you know right away that this class is responsible for performing a single task or process—often something more complex than what should sit in a controller or model.

The beauty of a well-named service is that any developer can open up your code, see PaymentProcessor.call and immediately know what it does. This kind of clarity helps people onboard quickly and prevents code from sprawling across controllers and models.

Example:

class PaymentProcessor
  def initialize(order)
    @order = order
  end

  def call
    charge_credit_card
    send_receipt
    update_order_status
  end
end

By pulling logic into a service like PaymentProcessor, you make your intent clear. You’re telling the next developer: “This is how we handle payments, and here’s the class that does it.” Services focus responsibility, making changes straightforward and reducing the need to hunt through controllers or models.

Concerns: Keep It Focused, Keep It Obvious

Concerns are a powerful way to share behavior between models and controllers. But the key to making concerns maintainable is keeping them focused. Ruby is an expressive language, and the beauty of expressive code is that, when done well, it can be read like English. If you use concerns correctly, a developer should be able to understand your intention just by reading the name.

For example, if you have a concern called Trackable, a developer should be able to see include Trackable in the model and immediately know what it does. “Ah, this model tracks something,” they’ll think. The idea is that they shouldn’t have to dig into the concern itself to understand the behavior.

Example:

module Trackable
  extend ActiveSupport::Concern

  included do
    before_create :set_tracking_code
  end

  def set_tracking_code
    self.tracking_code = SecureRandom.uuid
  end
end

The goal is clarity. When the next developer opens up your model and sees include Trackable, they should instantly know what’s going on. Keep your concerns focused, and name them well, so they’re as self-explanatory as possible.

Form Objects: Managing Complex Forms with Clarity

Form Objects help organize your logic when you have forms that don't map neatly to a single model. By encapsulating form validation and submission logic in a dedicated class, you not only make your controllers simpler but also make the intent behind the form clearer.

When the next developer comes in and sees UserRegistrationForm, they’ll know exactly where to go to find the logic that handles user registration. Form Objects give you a clean way to handle multi-step forms or forms involving multiple models, keeping your app’s logic easy to navigate.

Example:

class UserRegistrationForm
  include ActiveModel::Model

  attr_accessor :user, :address

  validates :user, :address, presence: true

  def save
    return false unless valid?

    ActiveRecord::Base.transaction do
      user.save!
      address.save!
    end
    true
  rescue ActiveRecord::RecordInvalid
    false
  end
end

With a well-defined form object, you keep your controller skinny and your logic centralized, making future changes easy to understand and maintain.

Policy Objects: Keeping Authorization Clear and Centralized

Authorization logic is crucial but can easily become scattered across controllers and models. Enter Policy Objects. They keep your authorization logic centralized, so it’s easy to manage and extend. You don't want the next developer trying to figure out who can update what by piecing together if statements in your controllers.

If you’re using the CanCanCan gem, this logic gets bundled into an ability.rb file. This file defines what a user can and cannot do, and it centralizes all authorization rules in one place.

Example:

class Ability
  include CanCan::Ability

  def initialize(user)
    if user.admin?
      can :manage, :all
    else
      can :read, Post
      can :update, Post, user_id: user.id
    end
  end
end

With Policy Objects or a gem like CanCanCan, authorization is easy to find and easy to update. The next developer can go straight to ability.rb and know exactly who has access to what. This makes maintenance straightforward and reduces the risk of scattered logic.

Query Objects: Keep Your Database Logic Organized

Complex database queries can become unwieldy when they’re embedded directly in models. Query Objects help you isolate this logic in one place, making it easier to reuse and test. One common use case is when you need to search across multiple fields. Instead of cluttering your model with complex query logic, you can move it into a query object.

Using a gem like pg_search, which provides full-text search capabilities, you can encapsulate these search queries cleanly in a Query Object.

Example:

class UserSearch
  def initialize(query)
    @query = query
  end

  def call
    User.pg_search(@query)
  end
end

This makes it clear that UserSearch handles all the complexity of searching users. The next developer won’t need to decipher long query chains in the model—they’ll just use the query object and know exactly where to look if they need to modify it.

Interactors: Managing Multi-Step Processes and State Machines

When a process involves multiple steps—like creating an order, sending notifications, and updating the inventory—you need something more structured than a basic service. Interactors are great for handling these kinds of workflows. They allow you to chain actions together and handle failures gracefully. When combined with a state machine, you can also keep track of complex business processes that transition between different states.

State machines help ensure that your app moves smoothly between predefined states. For example, an order might start as pending, move to paid, and finally to shipped.

Example:

class CreateOrder
  include Interactor

  def call
    order = Order.new(order_params)

    if order.save
      context.order = order
      send_notification
    else
      context.fail!(message: "Order could not be created")
    end
  end

  private

  def send_notification
    OrderMailer.confirmation(context.order).deliver_now
  end
end

By handling multi-step processes with an Interactor, you make it clear where each step of the workflow lives. And if the workflow involves transitioning between different states (like with a state machine), the next developer will understand the lifecycle of that process without needing to sift through scattered logic.

Engines: Modularize Your Features

As your app grows, some features become big enough to be self-contained. Engines allow you to modularize large chunks of functionality. They’re perfect for features like admin dashboards, payment systems, or any part of your app that can live as a mini-Rails app inside your main application.

By breaking large features into engines, you reduce the complexity of your main app, and the next developer can work on that feature in isolation. Engines help keep large, distinct features modular, so they don’t clutter up your main app. The next developer will be able to dive into the engine and understand the feature without wading through unrelated code.

Maintainable Code is Understandable Code

The best Rails applications are built for change. That means writing code that’s easy to understand, easy to navigate, and easy to extend. The right abstraction makes all the difference. When the next developer comes along, they should be able to see at a glance where the logic lives and why it’s there.

Using Services to isolate business logic, Concerns to share behavior without bloating your models, Form Objects to simplify complex forms, Policy Objects to centralize authorization, Query Objects to manage complex queries, Interactors to organize workflows, and Engines to modularize large features—all of these patterns make your code easier to maintain.

When you follow these patterns, you don’t just keep your code clean—you make it clear and approachable for the next developer who comes along. And that’s the key to building maintainable Rails apps that stand the test of time.