news.rb

class News < ActiveRecord::Base
  has_many :quotes, :autosave=>true, :dependent=>:destroy, :order => "position"

  MAX_QUOTES = 3

  before_save :remove_excess_quotes

  protected

  def remove_excess_quotes
    if self.quotes.count > MAX_QUOTES
      self.quotes.destroy_all ["position > ?", MAX_QUOTES] # nisam siguran da li ["",...] sintaksa radi za destroy_all - trebalo bi, ako ne radi samo napravi obican string
    end
  end
end

quote.rb

class Quote < ActiveRecord::Base
  belongs_to :news

  # https://github.com/swanandp/acts_as_list
  # pogledaj i sam source da vidis koje sve metode ima:
  # https://github.com/swanandp/acts_as_list/blob/master/lib/acts_as_list/active_record/acts/list.rb
  acts_as_list :scope => :news
end