feat: decrement inventory operation added

Support for this operation in Omni will make it easier to control double buying.

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-*
+* Support for auto-decrement of stock on payment of `Order` using the `inventory_decremented` flag
 
 ### Changed
 

src/budy/models/order.py

@@ -195,6 +195,12 @@ class Order(bundle.Bundle):
         redeeming of the vouchers associated with the current order"""
     )
 
+    inventory_decremented = appier.field(
+        type = bool,
+        observations = """Controls if the inventory stock levels
+        have been decrement according to current order's lines"""
+    )
+
     lines = appier.field(
         type = appier.references(
             "OrderLine",
@@ -962,6 +968,7 @@ def mark_paid_s(self):
         self.paid = True
         self.date = time.time()
         self.set_reference_f_s()
+        self.decrement_inventory_s()
         self.save()
 
     @appier.operation(name = "Unmark Paid")
@@ -1061,6 +1068,22 @@ def fix_closed_s(self):
         if not self.is_closed(): return
         self.close_lines_s()
 
+    @appier.operation(
+        name = "Decrement Inventory",
+        description = """Decrements the inventory stock levels
+        according to order lines""",
+        parameters = (("Force", "force", bool, False),),
+        level = 2
+    )
+    def decrement_inventory_s(self, force = False):
+        if self.inventory_decremented and not force: return
+        for line in self.lines:
+            line.merchandise.quantity_hand =\
+                max(line.merchandise.quantity_hand - line.quantity, 0)
+            line.merchandise.save()
+        self.inventory_decremented = True
+        self.save()
+
     @appier.operation(
         name = "Import Omni",
         parameters = (