Question

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 solution to do this, but what would be the most elegant way of doing this?

Answer 1

I think what you really want is a polymorphic association.

Ryan does a great job explaining them in Railscast #154.

class Followup < ActiveRecord::Base
  belongs_to :followupable, :polymorphic => true
end

class Post < ActiveRecord::Base
  has_many :followups, :as => :followupable
end

class Comment < ActiveRecord::Base
  has_many :followups, :as => :followupable
end