comment.rb

class Comment < ActiveRecord::Base
  belongs_to :post
end

irb

test_proxy djsun$ script/console
Loading development environment (Rails 2.3.4)
>> c = Comment.new
=> #<Comment id: nil, body: nil, post_id: nil, created_at: nil, updated_at: nil>
>> c.post
=> nil
>> !c.post
=> true
>> c.post ? "t" : "f"
=> "f"
>> c.post.nil?
=> true

post.rb

class Post < ActiveRecord::Base
 has_many :comments 
end