gistfile1.rb

class Promotion::Rules::OnePerUser < PromotionRule

  def eligible?(order)
    used_promotion_credits(order).empty?
  end
  
  def used_promotion_credits(order)
    p = PromotionCredit.arel_table
    o = Order.arel_table

    if order.user.anonymous?
      user_condition = o[:email].eq order.email
    else
      user_condition = o[:user_id].eq order.user_id
    end
    
    this_promo       = p[:source_id].eq(promotion_id)
    not_this_order   = o[:id].eq(order.id).not
    finalized_orders = o[:state].eq(Order::FINALIZED_STATES)
    by_this_user     = user_condition

    # checking if this promotion is used by the user in another order.
    PromotionCredit.includes(:order).where(this_promo, not_this_order, finalized_orders, by_this_user)
  end

end