Initializer not getting called?

gistfile1.rb

module Thinger
  extend ActiveSupport::Concern

  module ClassMethods
    def acts_as_something
      class_eval do
        has_many :assignments
        has_many :things, :through => :assignments

        attr_accessible :things
        after_initialize :set_default_thing
        validate :must_have_at_least_one_thing
      end
    end
  end

  # This method is not getting called, apparently?
  def set_default_things
    if (things.size == 0)
      things << Thing.find_or_create_new(:things_name)
    end
  end  
end

# This test fails:
it "should set the 'things_name' thing on initialization if things are empty" do
  no_thing_doohickey = Factory(:doohickey)
  no_thing_doohickey.has_thing?(:things_name).should be_true
end

# But it works in console just fine:
irb(main):002:0> @d = Doohickey.new
...
irb(main):003:0> @d.things
=> [#<Thing id: 1, name: "things_name", description: nil>]