Model-View-Controller Pattern

Implementing Rails-like Controllers in a No-Rails App

In its essence, a controller class in Rails is a coordinator. It accepts the incoming HTTP request and builds the data required by the view using the domain models. This post shows a highly simplified implementation of the controller pattern for a better understanding of Rails controllers.

9 min read

This is the fourth article in the series on building a web application in Ruby without using Rails. In the previous article, we built a working router, and this one shows how to implement a simple controller structure just like Rails in only four lines of Ruby.

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Our Goal: To build a highly simplified implementation of the controller pattern to better understand Rails controllers.

If you could take only one lesson from this post, it's this:

The incoming HTTP request doesn't hit your Rails controller's action method automagically out of thin air (something I used to think a while ago when I wrote ASP.NET MVC controllers), but there's a bunch of framework code behind the scenes that receives the request from the app server like Puma, processes it, creates an instance of the controller (just like any other class), and calls the action method on it. Then it takes the response returned by the action, processes it, and sends it back to the app server, which returns it to the browser.

If the last statement made you curious enough to dig deeper and trace an incoming HTTP request's path to the Rails controller, check out the following post after your read the current article.

How a Ruby Method Becomes a Rails Action: Part One (Metal)
In this post, we will explore how a simple Ruby method, when added to a controller, becomes an action in Rails, ready to process incoming HTTP requests and send responses. We’ll also trace the path of an incoming HTTP request to a Rails controller action.
Write Software, WellAkshay Khot

In its essence, a controller class in Rails is a coordinator. It accepts the incoming HTTP request and builds the data required by the view using the domain models. I hope that you'll have a much better understanding and appreciation for Rails controllers after reading this lesson.

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If you just want to see the final source code, I've uploaded it on GitHub.

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Rails Companion
Akshay Khot

I run an independent software studio/agency specializing in Ruby on Rails. This blog is my attempt to spread the joy of programming in Ruby and building web apps in Rails with the world.