class Followup < ActiveRecord::Base
  belongs_to :post
  belongs_to :comment
end

This model needs to only have either a post or a comment, but only one of the two.

Here's the rspec for what I'm trying to do:

  it "should be impossible to have both a comment and a post" do
    followup = Followup.make
    followup.comment = Comment.make
    followup.should be_valid
    followup.post = Post.make
    followup.should_not be_valid
  end

I can see a bunch of ways of doing this, but what would be the most elegant way of doing this?