gistfile1.rb

class Posts < ActiveRecord::Base
  belongs_to :user
  scope :extra, where(:extra => true)

  def self.optional_scope(param = false)    
    if param
      scope = where(:something => 'else')
    else
      scope = self
    end
    scope
  end
end

# working use
current_user.posts.optional_scope(true).extra.each { .. } current_user.posts.optional_scope(false).extra.each { .. }

# falling over because Class is returned
current_user.posts.optional_scope(false).each { .. }

# really ought to test that it's still maintaining the prior scopes in the chain...