Pricing bug fix (#4422)

* Control when a new PartPricing object can be created

- Prevent this when calling from an on_delete signal
- There is an edge case where deleting a part triggers a series of on_delete signals inside an atomic transaction
- When the new PartPricing object is created,

* Add unit testing:

- Ensure PartPricing gets created when a new StockItem is added
- Part.delete() works without error
- PartPricing instances are deleted also

* style fixes

InvenTree/part/models.py

@@ -1678,17 +1678,33 @@ class Part(InvenTreeBarcodeMixin, MetadataMixin, MPTTModel):
 
         return pricing
 
-    def schedule_pricing_update(self):
+    def schedule_pricing_update(self, create: bool = False):
         """Helper function to schedule a pricing update.
 
         Importantly, catches any errors which may occur during deletion of related objects,
         in particular due to post_delete signals.
 
         Ref: https://github.com/inventree/InvenTree/pull/3986
+
+        Arguments:
+            create: Whether or not a new PartPricing object should be created if it does not already exist
         """
 
-        pricing = self.pricing
-        pricing.schedule_for_update()
+        try:
+            self.refresh_from_db()
+        except Part.DoesNotExist:
+            return
+
+        try:
+            pricing = self.pricing
+
+            if create or pricing.pk:
+                pricing.schedule_for_update()
+        except IntegrityError:
+            # If this part instance has been deleted,
+            # some post-delete or post-save signals may still be fired
+            # which can cause issues down the track
+            pass
 
     def get_price_info(self, quantity=1, buy=True, bom=True, internal=False):
         """Return a simplified pricing string for this part.
@@ -2261,6 +2277,10 @@ class PartPricing(common.models.MetaMixin):
     def schedule_for_update(self, counter: int = 0):
         """Schedule this pricing to be updated"""
 
+        if not self.part or not self.part.pk or not Part.objects.filter(pk=self.part.pk).exists():
+            logger.warning("Referenced part instance does not exist - skipping pricing update.")
+            return
+
         try:
             if self.pk:
                 self.refresh_from_db()
@@ -3710,7 +3730,7 @@ def update_pricing_after_edit(sender, instance, created, **kwargs):
 
     # Update part pricing *unless* we are importing data
     if InvenTree.ready.canAppAccessDatabase() and not InvenTree.ready.isImportingData():
-        instance.part.schedule_pricing_update()
+        instance.part.schedule_pricing_update(create=True)
 
 
 @receiver(post_delete, sender=BomItem, dispatch_uid='post_delete_bom_item')
@@ -3721,7 +3741,7 @@ def update_pricing_after_delete(sender, instance, **kwargs):
 
     # Update part pricing *unless* we are importing data
     if InvenTree.ready.canAppAccessDatabase() and not InvenTree.ready.isImportingData():
-        instance.part.schedule_pricing_update()
+        instance.part.schedule_pricing_update(create=False)
 
 
 class BomItemSubstitute(models.Model):