gistfile1.rb

require 'active_support/inflector'

class Bar; end

def ControllerMixin(model)
  model_class = model.classify.constantize
  Module.new do
    define_method(:"current_#{model}") do
      model_class.find(session[:"current_#{model}_id"]) if session[:"current_#{model}_id"]
    end
  end
end

describe 'a ControllerMixin' do
  let(:controller) { double(session: {current_bar_id: 99}) }
  before { controller.extend ControllerMixin('bar') }

  it 'adds a current bar method' do
    Foo.should_receive(:find).with(99)
    controller.current_bar
  end
end