Fix MultipleObjectsReturned error on webhook endpoints (#1996)

# What this PR does
Sometimes `CustomButtonView` returns HTTP 500 with the following error:

```
apps.alerts.models.custom_button.CustomButton.MultipleObjectsReturned: get() returned more than one CustomButton -- it returned 3!
```

This PR fixes it by adding `.distinct()` to the `CustomButton` queryset
when retrieving an instance + does the same for `WebhooksView`.

## Which issue(s) this PR fixes
Related to grafana/oncall-private#1828

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Improve plugin authentication by @vadimkerr ([#1995](https://github.com/grafana/oncall/pull/1995))
+- Fix MultipleObjectsReturned error on webhook endpoints by @vadimkerr ([#1996](https://github.com/grafana/oncall/pull/1996))
 
 ## v1.2.27 (2023-05-23)
 

engine/apps/api/views/custom_button.py

@@ -64,7 +64,11 @@ def get_object_from_organization(self):
         pk = self.kwargs["pk"]
         organization = self.request.auth.organization
         try:
-            obj = organization.custom_buttons.filter(*self.available_teams_lookup_args).get(public_primary_key=pk)
+            obj = (
+                organization.custom_buttons.filter(*self.available_teams_lookup_args)
+                .distinct()
+                .get(public_primary_key=pk)
+            )
         except ObjectDoesNotExist:
             raise NotFound
 

engine/apps/api/views/webhooks.py

@@ -63,7 +63,7 @@ def get_object_from_organization(self):
         pk = self.kwargs["pk"]
         organization = self.request.auth.organization
         try:
-            obj = organization.webhooks.filter(*self.available_teams_lookup_args).get(public_primary_key=pk)
+            obj = organization.webhooks.filter(*self.available_teams_lookup_args).distinct().get(public_primary_key=pk)
         except ObjectDoesNotExist:
             raise NotFound
 

engine/common/api_helpers/mixins.py

@@ -197,6 +197,10 @@ class TeamFilteringMixin:
 
     @property
     def available_teams_lookup_args(self):
+        """
+        This property returns a list of Q objects that are used to filter instances by teams available to the user.
+        NOTE: use .distinct() after filtering by available teams as it may return duplicate instances.
+        """
         available_teams_lookup_args = []
         if not self.request.user.role == LegacyAccessControlRole.ADMIN:
             available_teams_lookup_args = [