NullUser On Rails

Aug 11, 2014 on Rails Ruby Authentication SoftwarePatterns

The Null Object Pattern is a popular software pattern that can be very useful as it can get rid of many nil checks in Ruby. Throughout your application, you may have many clauses like if current_user.admin?. If you require everyone who uses your application to log in, you won't have any problems, but if you don't, NoMethodError: undefined method 'admin?' for nil:NilClass. So how would you get rid of this error? Well, you could simply wrap your if current_user.admin? in a if current_user check, but you have to do this everywhere if current_user.admin? is called. This can be annoying and time consuming to track down and fix. Luckily, the aforementioned Null Object Pattern can get rid of all these checks. Depending on which authentication system you used, you may have to tweak this method a little bit but in Monban:

class ApplicationController < ActionController::Base
  include Monban::ControllerHelpers

  ...

  def current_user
    super || NullUser.new
  end
end

class NullUser
  def admin?
    false
  end
end

What's happening is in ApplicationController, current_user is being redefined to either the superclass of current_user, or a new instance of NullUser, if the superclass returns nil. The reason why this works is because in Ruby, when you include a module in your class, the chain of inheritance actually gets changed so it checks the module first, then the parent object, so feel free to call super as much as you want. Now, throughout the application, you won't have to change anything, and you won't get that ugly NoMethodError message.

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