[fix] Avoid N+1 queries when deleting VpnClient instances #1221

Added select_related("vpn", "cert", "ip") to VpnClient querysets
used for per-instance deletion to prevent N+1 database queries
triggered by the post_delete signal handler.

Related to #1221

Author: mn-ram

openwisp_controller/config/api/serializers.py

@@ -202,7 +202,9 @@ def _update_config(self, device, config_data):
                 with transaction.atomic():
                     vpn_list = config.templates.filter(type="vpn").values_list("vpn")
                     if vpn_list:
-                        for vpnclient in config.vpnclient_set.exclude(
+                        for vpnclient in config.vpnclient_set.select_related(
+                            'vpn', 'cert', 'ip'
+                        ).exclude(
                             vpn__in=vpn_list
                         ).iterator():
                             vpnclient.delete()

openwisp_controller/config/base/config.py

@@ -339,7 +339,9 @@ def manage_vpn_clients(cls, action, instance, pk_set, **kwargs):
                 # If the device is deactivated or in the process of deactivating, then
                 # delete all vpn clients and return.
                 with transaction.atomic():
-                    for vpnclient in instance.vpnclient_set.all().iterator():
+                    for vpnclient in instance.vpnclient_set.select_related(
+                        'vpn', 'cert', 'ip'
+                    ).iterator():
                         vpnclient.delete()
             return
 
@@ -373,7 +375,9 @@ def manage_vpn_clients(cls, action, instance, pk_set, **kwargs):
             # ones, have been fully added. At that point, we can identify and
             # delete VpnClient objects not linked to the final template set.
             with transaction.atomic():
-                for vpnclient in instance.vpnclient_set.exclude(
+                for vpnclient in instance.vpnclient_set.select_related(
+                    'vpn', 'cert', 'ip'
+                ).exclude(
                     template_id__in=instance.templates.values_list("id", flat=True)
                 ).iterator():
                     vpnclient.delete()