Tree query improvements (#3443)

* Allow part category table to be ordered by part count

* Add queryset annotation for part-category part-count

- Uses subquery to annotate the part-count for sub-categories
- Huge reduction in number of queries

* Update 'pathstring' property of PartCategory and StockLocation

- No longer a dynamically calculated value
- Constructed when the model is saved, and then written to the database
- Limited to 250 characters

* Data migration to re-construct pathstring for PartCategory objects

* Fix for tree model save() method

* Add unit tests for pathstring construction

* Data migration for StockLocation pathstring values

* Update part API

- Add new annotation to PartLocationDetail view

* Update API version

* Apply similar annotation to StockLocation API endpoints

* Extra tests for PartCategory API

* Unit test fixes

* Allow PartCategory and StockLocation lists to be sorted by 'pathstring'

* Further unit test fixes

InvenTree/stock/api.py

@@ -224,6 +224,13 @@ class StockLocationList(ListCreateAPI):
     queryset = StockLocation.objects.all()
     serializer_class = StockSerializers.LocationSerializer
 
+    def get_queryset(self, *args, **kwargs):
+        """Return annotated queryset for the StockLocationList endpoint"""
+
+        queryset = super().get_queryset(*args, **kwargs)
+        queryset = StockSerializers.LocationSerializer.annotate_queryset(queryset)
+        return queryset
+
     def filter_queryset(self, queryset):
         """Custom filtering: - Allow filtering by "null" parent to retrieve top-level stock locations."""
         queryset = super().filter_queryset(queryset)
@@ -293,6 +300,7 @@ class StockLocationList(ListCreateAPI):
 
     ordering_fields = [
         'name',
+        'pathstring',
         'items',
         'level',
         'tree_id',
@@ -1340,6 +1348,13 @@ class LocationDetail(RetrieveUpdateDestroyAPI):
     queryset = StockLocation.objects.all()
     serializer_class = StockSerializers.LocationSerializer
 
+    def get_queryset(self, *args, **kwargs):
+        """Return annotated queryset for the StockLocationList endpoint"""
+
+        queryset = super().get_queryset(*args, **kwargs)
+        queryset = StockSerializers.LocationSerializer.annotate_queryset(queryset)
+        return queryset
+
 
 stock_api_urls = [
     re_path(r'^location/', include([