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

---------

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

netbox/utilities/counters.py

@@ -0,0 +1,93 @@
+from django.apps import apps
+from django.db.models import F
+from django.db.models.signals import post_delete, post_save
+
+from netbox.registry import registry
+from .fields import CounterCacheField
+
+
+def get_counters_for_model(model):
+    """
+    Return field mappings for all counters registered to the given model.
+    """
+    return registry['counter_fields'][model].items()
+
+
+def update_counter(model, pk, counter_name, value):
+    """
+    Increment or decrement a counter field on an object identified by its model and primary key (PK). Positive values
+    will increment; negative values will decrement.
+    """
+    model.objects.filter(pk=pk).update(
+        **{counter_name: F(counter_name) + value}
+    )
+
+
+#
+# Signal handlers
+#
+
+def post_save_receiver(sender, instance, **kwargs):
+    """
+    Update counter fields on related objects when a TrackingModelMixin subclass is created or modified.
+    """
+    for field_name, counter_name in get_counters_for_model(sender):
+        parent_model = sender._meta.get_field(field_name).related_model
+        new_pk = getattr(instance, field_name, None)
+        old_pk = instance.tracker.get(field_name) if field_name in instance.tracker else None
+
+        # Update the counters on the old and/or new parents as needed
+        if old_pk is not None:
+            update_counter(parent_model, old_pk, counter_name, -1)
+        if new_pk is not None:
+            update_counter(parent_model, new_pk, counter_name, 1)
+
+
+def post_delete_receiver(sender, instance, **kwargs):
+    """
+    Update counter fields on related objects when a TrackingModelMixin subclass is deleted.
+    """
+    for field_name, counter_name in get_counters_for_model(sender):
+        parent_model = sender._meta.get_field(field_name).related_model
+        parent_pk = getattr(instance, field_name, None)
+
+        # Decrement the parent's counter by one
+        if parent_pk is not None:
+            update_counter(parent_model, parent_pk, counter_name, -1)
+
+
+#
+# Registration
+#
+
+def connect_counters(*models):
+    """
+    Register counter fields and connect post_save & post_delete signal handlers for the affected models.
+    """
+    for model in models:
+
+        # Find all CounterCacheFields on the model
+        counter_fields = [
+            field for field in model._meta.get_fields() if type(field) is CounterCacheField
+        ]
+
+        for field in counter_fields:
+            to_model = apps.get_model(field.to_model_name)
+
+            # Register the counter in the registry
+            change_tracking_fields = registry['counter_fields'][to_model]
+            change_tracking_fields[f"{field.to_field_name}_id"] = field.name
+
+            # Connect the post_save and post_delete handlers
+            post_save.connect(
+                post_save_receiver,
+                sender=to_model,
+                weak=False,
+                dispatch_uid=f'{model._meta.label}.{field.name}'
+            )
+            post_delete.connect(
+                post_delete_receiver,
+                sender=to_model,
+                weak=False,
+                dispatch_uid=f'{model._meta.label}.{field.name}'
+            )