A module version of the fast specs stuff from http://ariejan.net/2011/10/13/fast-specs-run-your-specs-in-less-than-1-second

post.rb

class Post < ActiveRecord::Base
  include Summarizeable
  belongs_to :author
  alias :body :summarizeable
end

summarizeable_spec.rb

# spec/models/summarizeable_spec.rb
require 'spec/helper'

describe Summarizeable do
  context "summary" do
    it "should return the summary" do
      thing = SummaryStub.new
      thing.text_to_summarize = "This is what I expect.\nI don't expect this!"
      assert_equal "This is what I expect.", thing.summary
    end
  end
end

summary.rb

module Summarizeable
  def summary(delimiter="\n")
    matcher = /#{Regexp.escape(delimiter)}/i
    @summary = if summarizeable =~ matcher
                 summerizeable.split(matcher).first.strip
               else
                 summerizeable
               end
  end
end

summary_stub.rb

class SummaryStub
  include Summarizeable
  attr_accessible :text_to_summarize
  alias :text_to_summarize :summarizeable
end

Summarizeable#summary creates the instance variable @summary. Is that appropriate behavior for a module?

Sometimes. Probably not in this instance.