gistfile1.rb

# With macro
has_many :comments, :foreign_key => 'commentable_id', :conditions => "commentable_type = 'User'", :dependent => :destroy

# Without macro
def comments
  Comment.all(:conditions => {:commentable_id => self.id, :commentable_type => 'User'})
end

before_destroy :remove_associated_comments

def remove_associated_comments
   Comment.destroy_all({:commentable_id => self.id, :commentable_type => "User"})
end