Prevent serial number for stock item with quantity > 1

InvenTree/static/css/inventree.css

@@ -86,3 +86,7 @@
     align-items: stretch;
     display: flex;
 }
+
+.help-inline {
+    color: #A11;
+}

InvenTree/stock/models.py

@@ -73,6 +73,13 @@ class StockItem(models.Model):
             # TODO - Find a test than can be perfomed...
             pass
 
+        # Serial number cannot be set for items with quantity greater than 1
+        if not self.quantity == 1 and self.serial:
+            raise ValidationError({
+                'quantity': _("Quantity must be set to 1 for item with a serial number"),
+                'serial': _("Serial number cannot be set if quantity > 1")
+            })
+
     def get_absolute_url(self):
         return '/stock/item/{id}/'.format(id=self.id)