6347 Cache the number of each component type assigned to devices/VMs (#12632)

---------

Co-authored-by: Jeremy Stretch <jstretch@netboxlabs.com>

netbox/utilities/management/commands/calculate_cached_counts.py

@@ -0,0 +1,52 @@
+from collections import defaultdict
+
+from django.core.management.base import BaseCommand
+from django.db.models import Count, OuterRef, Subquery
+
+from netbox.registry import registry
+
+
+class Command(BaseCommand):
+    help = "Force a recalculation of all cached counter fields"
+
+    @staticmethod
+    def collect_models():
+        """
+        Query the registry to find all models which have one or more counter fields. Return a mapping of counter fields
+        to related query names for each model.
+        """
+        models = defaultdict(dict)
+
+        for model, field_mappings in registry['counter_fields'].items():
+            for field_name, counter_name in field_mappings.items():
+                fk_field = model._meta.get_field(field_name)        # Interface.device
+                parent_model = fk_field.related_model               # Device
+                related_query_name = fk_field.related_query_name()  # 'interfaces'
+                models[parent_model][counter_name] = related_query_name
+
+        return models
+
+    def update_counts(self, model, field_name, related_query):
+        """
+        Perform a bulk update for the given model and counter field. For example,
+
+            update_counts(Device, '_interface_count', 'interfaces')
+
+        will effectively set
+
+            Device.objects.update(_interface_count=Count('interfaces'))
+        """
+        self.stdout.write(f'Updating {model.__name__} {field_name}...')
+        subquery = Subquery(
+            model.objects.filter(pk=OuterRef('pk')).annotate(_count=Count(related_query)).values('_count')
+        )
+        return model.objects.update(**{
+            field_name: subquery
+        })
+
+    def handle(self, *model_names, **options):
+        for model, mappings in self.collect_models().items():
+            for field_name, related_query in mappings.items():
+                self.update_counts(model, field_name, related_query)
+
+        self.stdout.write(self.style.SUCCESS('Finished.'))