gistfile1.rb

# your_object.rb
class YourObject < ActiveRecord::Base
  def your_method
    self.your_association << TestObject.new
  end
end

# your_object_spec.rb
it "should test your_method" do
  lambda { your_object.your_method }.should change(your_object.your_association, :size)   # => no change, test fails....
end