let.rb

# app/concerns/let.rb
# Ripped from Rspec 2.0 beta.12
# I've found it so useful in Rspec, I made it into a module. Controller code with shared
# code seem to have a lot of use for this. For example, in inherited_resources, you would
# typically override model, parent, etc. to configure it. You would typically memomize it
# as well. With this, you can use a more compact let() syntax.

# (Note: This code will only work in Rails 3)

module Let
  extend ActiveSupport::Concern

  included do
    extend Let::ClassMethods
  end

  private

  def __memoized # :nodoc:
    @__memoized ||= {}
  end

  module ClassMethods
    def let(name, &block)
      define_method(name) do
        __memoized[name] ||= instance_eval(&block)
      end
      protected(name)
    end
  end

end

let_example_controller.rb

module Admin
  class CustomersController < ApplicationController
    include Controllers::AdminSharedCode

    let(:model) { Customers.by_owner(current_user) }
    let(:target_ids) { params[:targets] }
  end
end