Rails validations app specific problems

Hi all,

I have a unique workflow where it's making the use of validations
difficult.

My app is for internal use in my company and I have a funky
implementation (inspired by the e-commerce Railscasts episodes) of a
cart. I have products which when I "add to cart" creates an order with a
state of "incomplete". The product is a line item of my order. Hope that
makes sense.

Even though this is a little complicated, it works well (even though it
may not be the best implementation - suggestions are welcome though!)

Now the problem is when I go to checkout and complete my order I want to
validate certain things, like if my customer's order number is present
(validates_presence_of). This should be easy, but since the order has
ALREADY been created I need to validate the presence of an attribute.

Also, I can't just do this:

[code]
validates_presence_of :customer_order_number, :on => :update
[/code]

Because it generates a validation error on my '/cart' page which is
where I can go and edit the product quantities for the client order.

**I'm sure the solution lies in validates_presence_of with the :if
condition, but I just can't figure out how to make it work.**

Below is my related app code.

Orders Controller:

[code]
  def update
    @order = Orders.find(params[:id])

    respond_to do |format|
        if @order.update_attributes(params[:order])
          flash[:notice] = 'Your cart has been updated.'
          format.html { redirect_to( current_cart_url ) }
          format.xml { head :ok }
        else
          format.html { redirect_to( current_cart_url ) }
          format.xml { render :xml => @order.errors, :status =>
:unprocessable_entity }
        end
    end

  end

  def cart
    create_current_order unless current_order
    @order = current_order
  end

  def checkout
    @order = current_order
  end

  def complete_order
    @order = current_order

    respond_to do |format|
      if @order.update_attributes(params[:order])
        @order.complete #sets order completed at date and state to
'complete'
        flash[:notice] = 'Thank you! Your order is being processed.'
        format.html { redirect_to( products_path ) }
        format.xml { head :ok }
      else
        format.html { render :action => 'checkout' }
        format.xml { render :xml => @order.errors, :status =>
:unprocessable_entity }
      end
    end

  end
[/code]

I know this is kind of tough to understand, please let me know if you
need more specifics or code. Thank you!

-Tony

The :if option is just the name of an instance method that returns
whether the validation should run (eg if the status has some value or
something like that)

Fred