demeter.rb

class User < ActiveRecord::Base

  # ...

  def method_missing method, *args
    if method.to_s.starts_with? 'find'
      reading_lists.find *args
    else
      super
    end
  end
end

# Instead of user.reading_list.find params[:reading_list_id]
# Do user.find_reading_list params[:reading_list_id]
# It just needs generalising