Fix error when deleting observer subsciption

docs/CHANGELOG.rst

@@ -6,6 +6,15 @@ All notable changes to this project are documented in this file.
 This project adheres to `Semantic Versioning <http://semver.org/>`_.
 
 
+==========
+Unreleased
+==========
+
+Fixed
+-----
+- Fix error when deleting observer subscription
+
+
 ===================
 42.2.0 - 2025-01-13
 ===================

resolwe/observers/models.py

@@ -309,16 +309,20 @@ def subscribe(
                 )
                 self.observers.add(observer)
 
+    @transaction.atomic
     def delete(self):
         """Delete the given subscription.
 
         Delete all observers with no remaining subscriptions.
+
+        This method is marked atomic, so no new subsciptions are added to the observers
+        we are deleting in the meantime causing IntegrityError.
         """
-        # Find related observers with only one remaining subscription
-        # (it must be this one) and delete them first.
+        # First find observers with only this subscription and delete them.
         Observer.objects.annotate(subs=Count("subscriptions")).filter(
             subscriptions=self.pk, subs=1
         ).delete()
+        # Now delete the subscription itself.
         super().delete()
 
     @classmethod