with_options.rb

 with_options :order => 'created_at', :class_name => 'Comment' do |post|
    post.has_many :comments, :conditions => ['approved = ?', true], :dependent => :delete_all
    post.has_many :unapproved_comments, :conditions => ['approved = ?', false]
    post.has_many :all_comments
  end
#You can nest with_options blocks, and you can even use the same name for the block parameter each time. E.g.:
  class Product
    with_options :dependent => :destroy do |product|
      product.with_options :class_name => 'Media' do |product|
        product.has_many :images, :conditions => {:content_type => 'image'}
        product.has_many :videos, :conditions => {:content_type => 'video'}
      end
 
      product.has_many :comments
    end
  end