Logic fix for part filtering

InvenTree/part/api.py

@@ -149,6 +149,13 @@ class PartList(generics.ListCreateAPIView):
 
     - GET: Return list of objects
     - POST: Create a new Part object
+
+    The Part object list can be filtered by:
+        - category: Filter by PartCategory reference
+        - cascade: If true, include parts from sub-categories
+        - is_template: Is the part a template part?
+        - variant_of: Filter by variant_of Part reference
+        - assembly: Filter by assembly field
     """
 
     serializer_class = part_serializers.PartSerializer
@@ -291,23 +298,23 @@ class PartList(generics.ListCreateAPIView):
 
         cascade = str2bool(self.request.query_params.get('cascade', False))
 
-        if cat_id is not None:
-
-            if isNull(cat_id):
+        if cat_id is None:
+            # Top-level parts
+            if not cascade:
                 parts_list = parts_list.filter(category=None)
-            else:
-                try:
-                    cat_id = int(cat_id)
-                    category = PartCategory.objects.get(pk=cat_id)
 
-                    # If '?cascade=true' then include parts which exist in sub-categories
-                    if cascade:
-                        parts_list = parts_list.filter(category__in=category.getUniqueChildren())
-                    # Just return parts directly in the requested category
-                    else:
-                        parts_list = parts_list.filter(category=cat_id)
-                except (ValueError, PartCategory.DoesNotExist):
-                    pass
+        else:
+            try:
+                category = PartCategory.objects.get(pk=cat_id)
+
+                # If '?cascade=true' then include parts which exist in sub-categories
+                if cascade:
+                    parts_list = parts_list.filter(category__in=category.getUniqueChildren())
+                # Just return parts directly in the requested category
+                else:
+                    parts_list = parts_list.filter(category=cat_id)
+            except (ValueError, PartCategory.DoesNotExist):
+                pass
 
         # Ensure that related models are pre-loaded to reduce DB trips
         parts_list = self.get_serializer_class().setup_eager_loading(parts_list)