Simple null object

hash.rb

class CoursesController < ApplicationController
  expose(:course, attributes: :course_params)
  expose(:current_course) { current_user.courses.find_by_id(params[:id] || params[:course_id]) }
  expose(:first_of_each_month_of_course) {  (current_course.start_date.change(day: 1)..current_course.end_date).select{ |d| d.day == 1 }  }

  # ...
end

# Orginal code gives:
# line 4 of [PROJECT_ROOT]/app/controllers/courses_controller.rb: block in <class:CoursesController>
NoMethodError - undefined method `start_date' for nil:NilClass

hash2.rb

class CoursesController < ApplicationController
  expose(:course, attributes: :course_params)
  expose(:current_course) { id = params.fetch(:id) { params.fetch(:course_id) }
                            current_user.courses.find_by_id(id) }
  expose(:first_of_each_month_of_course) {  (current_course.start_date.change(day: 1)..current_course.end_date).select{ |d| d.day == 1 }  }
end

# Now code gives:
# line 3 of [PROJECT_ROOT]/app/controllers/courses_controller.rb: block in <class:CoursesController>
KeyError: key not found: :course_id

have_to_have_empty.rb

...looup gary's sentinel object

null_object_example.rb

class Client < ActiveRecord::Base
  belongs_to :pricing_level
end

class PricingLevel
  def self.find_default
    #...
  end
end

class Guest
  def name
    'Guest'
  end
  
  def pricing_level
    PricingLevel.find_default
  end
end