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
@@ -291,13 +298,13 @@ class PartList(generics.ListCreateAPIView):
 
         cascade = str2bool(self.request.query_params.get('cascade', False))
 
-        if cat_id is not None:
+        if cat_id is None:
-
+            # Top-level parts
-            if isNull(cat_id):
+            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