gistfile1.rb

class List < ActiveRecord::Base
  acts_as_list :order => :position
  validates_presence_of :name, :link
  validates_numericality_of :position, :only_integer => true , :allow_nil => false

  protected
  def validate
    if position < 1
      errors.add(:position , " must be greater than 0")
    end
   end
end