Cepega

class Item < ActiveRecord::Base
  belongs_to :user
  before_validation :set_user_increment_id, :on => :create
  validates :user_increment_id, :presence => true, :uniqueness => {:scope => :user_id}
  private

  def set_user_increment_id
    self.user_increment_id = self.class.where(:user_id => user_id).last.user_increment_id + 1 rescue 1
  end
end