captured sparks

One of the things that had me confused early on when programming in Ruby was when to use “self” for a method and when not to.

The easiest way to summarise, I think, is to say that self.method should be used when referring to a method that operates on the class itself. method should be used when referring to a method that operates on an instance of the class.

The most common place this is seen in Rails application is in authentication systems. The method for logging a user into an application is usually written as:

Class User < ActiveRecord::Base

  def self.authenticate(user, password)
    ...
  end

end

This is then called in the controller as:

Class SessionsController < ActionController::Base

  def new
    @user = User.authenticate(params[:session])
  end

end

In contrast, methods can then be called on the @user object once the user has logged in. For example, to check if the user is an administrator, the following method could be used in our User class:

def is_admin?
  ...
end

This could then be called within the view, perhaps to display certain restricted information:

<%= if @user.is_admin? %>

As a complete aside, I love how the above code demonstrates the readability of Ruby. There is no doubt as to what that statement means.