Populate subform from unsaved parent form

Hello,

I'm getting my feet wet with Zoho creator. I want to create a rudimentary parts management system that will have records for manufacture, testing, orders, RMA, etc.

I am trying to create a form for orders, consisting of a PO#, date, and customer (which is a lookup from the customer form).

I have an order item subform that includes PO lookup from the parent form, a lookup for the item part number from the 'item' form, and quantity.

The problem is, the PO# can't be looked up until the parent form is saved, then i have to go back, edit, and add the subform items.

I am sure I'm missing something. Is there a way for the subform items to access the PO from the unsaved parent form? Or, even better, do subform entries automatically inhererit some ID from the parent entry, that can later be used to look up all the entries for a PO, or which can get passed to a later shipping form and look up the PO a PCB was shipped against?

Many thanks,

Dave