Cascade delete in Ruby ActiveRecord models?

Yes. On a Rails' model association you can specify the :dependent option, which can take one of the following three forms:

• :destroy/:destroy_all The associated objects are destroyed alongside this object by calling their destroy method
• :delete/:delete_all All associated objects are destroyed immediately without calling their :destroy method
• :nullify All associated objects' foreign keys are set to NULL without calling their save callbacks

Note that the :dependent option is ignored if you have a :has_many X, :through => Y association set up.

So for your example you might choose to have a post delete all its associated comments when the post itself is deleted, without calling each comment's destroy method. That would look like this:

class Post < ActiveRecord::Base
  validates_presence_of :body, :title
  has_many :comments, :dependent => :delete_all
end

Update for Rails 4:

In Rails 4, you should use :destroy instead of :destroy_all.

If you use :destroy_all, you'll get the exception:

The :dependent option must be one of [:destroy, :delete_all, :nullify, :restrict_with_error, :restrict_with_exception]