Sohan's Blog

Living the Developer's Life

Implementing Template Method in Rails Controllers Using Module and Mixin

All rails controllers are subclasses of the ApplicationController class. A typical controller class declaration will look like the following-

class LoginController < ApplicationController
#the actions go here
end

With this basic information, I would like to state the problem first.


The Problem/Job in Hand

Add an action switch_project to all the controllers (> 20) of the ScrumPad code base. The implementations of the switch_project method will be same for all the controllers only other than the fact that, the switching destination will be different.


Analysis

Placing the switch_project action in the ApplicationController would be the best option. But, the methods of application controller are not accessible as actions. So, the following won’t work


class ApplicationController
def switch_project
if(is_valid_switch?)
logger.debug(“A switch is taking place”)
destination = get_destination(new_project_id)
redirect_to destination
end
end
end

if you hit http://server/login/switch_project you will get a server side error. However, if you instead place the switch_project inside the LoginController, it will work fine. But, of course at a cost. You need to copy/paste this method 20+ times as there are 20+ controllers with the same need! Horrible!


Again, as I said, the get_destination(new_project_id) is the only part that will be different for each of the controllers. So, we definitely find a template method here.


The Solution

If you need an introduction about Module and Mixin in Ruby, please read here at ruby-doc. We are going to use Mixin to implement the desired solution, efficiently.

So, I put the switch_project method in a module called, ProjectSwitchModule inside a new file at app/controllers/project_switch_module.rb like this-


module ProjectSwitchModule
def switch_project
if(is_valid_switch?)
logger.debug(“A switch is taking place”)
destination = get_destination(new_project_id)
redirect_to destination
end
end
def is_valid_switch?
#I determine if the switch is valid at this method and return boolean
end
end

To make it available to all my controllers, I include this module in just in the ApplicationController in the following way-


require ‘project_switch_module’
class ApplicationController
include ProjectSwitchModule
end

Also, to provide controller specific implementation of the get_destination(project_id) method, I am just writing this method in each of the controllers in the following way-


class LoginController
private
def get_destination(project_id)
#custom destination logic for LoginController
end
end
class MessageController
private
def get_destination(project_id)
#custom destination logic for MessageController
end
end

Now, if I invoke http://server/login/switch_project or http://server/message/switch_project I will get the result of the switch_project action. So, this gives us an elegant way to follow design patterns for efficient implementation. It will save a lot of my time in future when I need to change the switch_project method, since I just need to change at a single place instead of 20s.


Afterthoughts

If, for a controller the switch_project needs to be different from the template at the module, it is achieved simply by overriding the switch_project inside the controller. No worries!


I will appreciate any feedback on this article.

Comments

Sohan
Yes, in Ruby code block is a good way to implement templates. In most inumerator related templates, the library made extensive use of code blocks.
Ashrafuzzaman
Module and Mixin is an interesting way of implementing template.
“Code Block” can be one other alternative way.
Arif
Very good example of code reuse and single point of change.Thanks
Sohan
Thanks for your comment!
Anonymous
good post.