django: Use .exists() instead of .count() when possible.

tools/semgrep.yml

@@ -209,3 +209,27 @@ rules:
       Annotated types containing zerver.lib.typed_endpoint.ApiParamConfig should not be nested inside Optional. Use Annotated[Optional[...], zerver.lib.typed_endpoint.ApiParamConfig(...)] instead.
     languages: [python]
     severity: ERROR
+
+  - id: exists-instead-of-count
+    patterns:
+      - pattern-either:
+          - pattern: ... .count() == 0
+          - pattern: |
+              if not ... .count():
+                  ...
+    message: 'Use "not .exists()" instead; it is more efficient'
+    languages: [python]
+    severity: ERROR
+
+  - id: exists-instead-of-count-not-zero
+    patterns:
+      - pattern-either:
+          - pattern: ... .count() != 0
+          - pattern: ... .count() > 0
+          - pattern: ... .count() >= 1
+          - pattern: |
+              if ... .count():
+                  ...
+    message: 'Use ".exists()" instead; it is more efficient'
+    languages: [python]
+    severity: ERROR

zerver/actions/invites.py

@@ -134,7 +134,7 @@ def too_many_recent_realm_invites(realm: Realm, num_invitees: int) -> bool:
         not estimated_sent["messages"]
         # Only after we've done the rough-estimate check, take the
         # time to do the exact check:
-        and Message.objects.filter(realm=realm, sender__is_bot=False).count() == 0
+        and not Message.objects.filter(realm=realm, sender__is_bot=False).exists()
     ):
         warning_flags.append("no-messages-sent")
 

zerver/actions/realm_domains.py

@@ -103,7 +103,7 @@ def do_remove_realm_domain(
         },
     )
 
-    if RealmDomain.objects.filter(realm=realm).count() == 0 and realm.emails_restricted_to_domains:
+    if not RealmDomain.objects.filter(realm=realm).exists() and realm.emails_restricted_to_domains:
         # If this was the last realm domain, we mark the realm as no
         # longer restricted to domain, because the feature doesn't do
         # anything if there are no domains, and this is probably less

zerver/lib/push_notifications.py

@@ -484,7 +484,7 @@ def send_android_push_notification(
                 logger.warning("GCM: Got canonical ref but it already matches our ID %s!", reg_id)
             elif not DeviceTokenClass._default_manager.filter(
                 token=new_reg_id, kind=DeviceTokenClass.GCM
-            ).count():
+            ).exists():
                 # This case shouldn't happen; any time we get a canonical ref it should have been
                 # previously registered in our system.
                 #

zerver/lib/send_email.py

@@ -454,7 +454,7 @@ def clear_scheduled_emails(user_id: int, email_type: Optional[int] = None) -> No
 
     for item in items:
         item.users.remove(user_id)
-        if item.users.all().count() == 0:
+        if not item.users.all().exists():
             # Due to our transaction holding the row lock we have a guarantee
             # that the obtained COUNT is accurate, thus we can reliably use it
             # to decide whether to delete the ScheduledEmail row.

zerver/management/commands/delete_realm.py

@@ -39,7 +39,7 @@ realms used for testing; consider using deactivate_realm instead."""
             customer = get_customer_by_realm(realm)
             if customer and (
                 customer.stripe_customer_id
-                or CustomerPlan.objects.filter(customer=customer).count() > 0
+                or CustomerPlan.objects.filter(customer=customer).exists()
             ):
                 raise CommandError("This realm has had a billing relationship associated with it!")
 

zerver/migrations/0237_rename_zulip_realm_to_zulipinternal.py

@@ -13,7 +13,7 @@ def rename_zulip_realm_to_zulipinternal(
     Realm = apps.get_model("zerver", "Realm")
     UserProfile = apps.get_model("zerver", "UserProfile")
 
-    if Realm.objects.count() == 0:
+    if not Realm.objects.exists():
         # Database not yet populated, do nothing:
         return
 

zerver/models.py

@@ -3569,7 +3569,7 @@ class Attachment(AbstractAttachment):
     scheduled_messages = models.ManyToManyField("ScheduledMessage")
 
     def is_claimed(self) -> bool:
-        return self.messages.count() > 0 or self.scheduled_messages.count() > 0
+        return self.messages.exists() or self.scheduled_messages.exists()
 
     def to_dict(self) -> Dict[str, Any]:
         return {

zerver/tests/test_message_edit.py

@@ -4639,10 +4639,9 @@ class EditMessageTest(EditMessageTestCase):
         )
 
         assert (
-            UserMessage.objects.filter(user_profile=cordelia, message__id=messages[2].id)
+            not UserMessage.objects.filter(user_profile=cordelia, message__id=messages[2].id)
             .extra(where=[UserMessage.where_unread()])
-            .count()
+            .exists()
-            == 0
         )
 
         # Now move to a weird state and confirm we get the normal topic moved message.
@@ -4712,10 +4711,9 @@ class EditMessageTest(EditMessageTestCase):
         )
 
         assert (
-            UserMessage.objects.filter(user_profile=cordelia, message__id=messages[4].id)
+            not UserMessage.objects.filter(user_profile=cordelia, message__id=messages[4].id)
             .extra(where=[UserMessage.where_unread()])
-            .count()
+            .exists()
-            == 0
         )