subform writing to another form

subform writing to another form

I am creating an inventory system for a lending library of recording equipment. It starts with a parent form for selecting the user borrowing equipment, and in a subform a lookup field pulls the item number from the equipment inventory form. I'm trying to update the availability of each item loaned in the inventory on_success of the parent form. I can get the first item selected in the subform to update to the inventory form, but not the second, third, etc. Below is my workflow that is not working.

inventory = equipment_inventory[ID == input.Items_for_Checkout.Item_Number];
for each record in input.Items_for_Checkout
{
inventory.Availability="Checked Out";
}


Thanks. Any help is appreciated.