Cascade delete in Ruby ActiveRecord models?

I was following the screencast on rubyonrails.org (creating the blog).

I have following models:

comment.rb

class Comment < ActiveRecord::Base
    belongs_to :post
    validates_presence_of :body # I added this
end

post.rb

class Post < ActiveRecord::Base
    validates_presence_of :body, :title
    has_many :comments
end

Relations between models work fine, except for one thing - when I delete a post record, I'd expect RoR to delete all related comment records. I understand that ActiveRecords is database independent, so there's no built-in way to create foreign key, relations, ON DELETE, ON UPDATE statements. So, is there any way to accomplish this (maybe RoR itself could take care of deleting related comments? )?