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/company/migrations/0026_auto_20201110_1011.py

@@ -23,7 +23,7 @@ def migrate_currencies(apps, schema_editor):
     for the SupplierPriceBreak model, to a new django-money compatible currency.
     """
 
-    logger.info("Updating currency references for SupplierPriceBreak model...")
+    logger.debug("Updating currency references for SupplierPriceBreak model...")
 
     # A list of available currency codes
     currency_codes = CURRENCIES.keys()

InvenTree/company/models.py

@@ -722,7 +722,7 @@ def after_save_supplier_price(sender, instance, created, **kwargs):
     if InvenTree.ready.canAppAccessDatabase() and not InvenTree.ready.isImportingData():
 
         if instance.part and instance.part.part:
-            instance.part.part.schedule_pricing_update()
+            instance.part.part.schedule_pricing_update(create=True)
 
 
 @receiver(post_delete, sender=SupplierPriceBreak, dispatch_uid='post_delete_supplier_price_break')
@@ -732,4 +732,4 @@ def after_delete_supplier_price(sender, instance, **kwargs):
     if InvenTree.ready.canAppAccessDatabase() and not InvenTree.ready.isImportingData():
 
         if instance.part and instance.part.part:
-            instance.part.part.schedule_pricing_update()
+            instance.part.part.schedule_pricing_update(create=False)