Optimize duplicate queries occurring in AlertGroupFilter (#1809)

# What this PR does

In `AlertGroupFilter` we currently have 11 duplicate queries which add
~1-2secs of unecessary request latency to `GET
/api/internal/v1/alertgroups` calls.
![Screenshot 2023-04-20 at 17 57
49](https://user-images.githubusercontent.com/9406895/233589341-de5e53ca-f10b-4038-ad68-a857d1643bf2.png)



The queries originate from the `queryset` callable arguments on several
of the fields of the `AlertGroupFilter` class. These callables basically
filter down their respective querysets to include only objects that
belong to the currently authenticated user's organization.

The duplicate queries are
- 2 queries to fetch integrations
- 2 queries to fetch escalation chains
- 10 queries to fetch users

At the moment, this PR is still a draft and doesn't work as intended.
It's based off of [a suggestion from this
discussion](https://github.com/carltongibson/django-filter/discussions/1572#discussioncomment-5676042)
in the `django-filter` repo. See [this DjangoCon
talk](https://youtu.be/e52S1SjuUeM?t=841) for more context.

---------

Co-authored-by: Ildar Iskhakov <Ildar.iskhakov@grafana.com>

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add 2, 3 and 6 hours silence options
 
+## Fixed
+
+- Optimize duplicate queries occurring in AlertGroupFilter by @joeyorlando ([1809](https://github.com/grafana/oncall/pull/1809))
+
 ## v1.2.15 (2023-04-24)
 
 ### Fixed

engine/apps/api/views/alert_group.py

@@ -53,6 +53,31 @@ def get_user_queryset(request):
     return User.objects.filter(organization=request.user.organization).distinct()
 
 
+class AlertGroupFilterBackend(filters.DjangoFilterBackend):
+    """
+    See here for more context on how this works
+
+    https://github.com/carltongibson/django-filter/discussions/1572
+    https://youtu.be/e52S1SjuUeM?t=841
+    """
+
+    def get_filterset(self, request, queryset, view):
+        filterset = super().get_filterset(request, queryset, view)
+
+        filterset.form.fields["integration"].queryset = get_integration_queryset(request)
+        filterset.form.fields["escalation_chain"].queryset = get_escalation_chain_queryset(request)
+
+        user_queryset = get_user_queryset(request)
+
+        filterset.form.fields["silenced_by"].queryset = user_queryset
+        filterset.form.fields["acknowledged_by"].queryset = user_queryset
+        filterset.form.fields["resolved_by"].queryset = user_queryset
+        filterset.form.fields["invitees_are"].queryset = user_queryset
+        filterset.form.fields["involved_users_are"].queryset = user_queryset
+
+        return filterset
+
+
 class AlertGroupFilter(DateRangeFilterMixin, ByTeamModelFieldFilterMixin, ModelFieldFilterMixin, filters.FilterSet):
     """
     Examples of possible date formats here https://docs.djangoproject.com/en/1.9/ref/settings/#datetime-input-formats
@@ -69,19 +94,19 @@ class AlertGroupFilter(DateRangeFilterMixin, ByTeamModelFieldFilterMixin, ModelF
     silenced_at = filters.CharFilter(field_name="silenced_at", method=DateRangeFilterMixin.filter_date_range.__name__)
     silenced_by = filters.ModelMultipleChoiceFilter(
         field_name="silenced_by_user",
-        queryset=get_user_queryset,
+        queryset=None,
         to_field_name="public_primary_key",
         method=ModelFieldFilterMixin.filter_model_field.__name__,
     )
     integration = filters.ModelMultipleChoiceFilter(
         field_name="channel_filter__alert_receive_channel",
-        queryset=get_integration_queryset,
+        queryset=None,
         to_field_name="public_primary_key",
         method=ModelFieldFilterMixin.filter_model_field.__name__,
     )
     escalation_chain = filters.ModelMultipleChoiceFilter(
         field_name="channel_filter__escalation_chain",
-        queryset=get_escalation_chain_queryset,
+        queryset=None,
         to_field_name="public_primary_key",
         method=ModelFieldFilterMixin.filter_model_field.__name__,
     )
@@ -90,21 +115,21 @@ class AlertGroupFilter(DateRangeFilterMixin, ByTeamModelFieldFilterMixin, ModelF
     )
     resolved_by = filters.ModelMultipleChoiceFilter(
         field_name="resolved_by_user",
-        queryset=get_user_queryset,
+        queryset=None,
         to_field_name="public_primary_key",
         method=ModelFieldFilterMixin.filter_model_field.__name__,
     )
     acknowledged_by = filters.ModelMultipleChoiceFilter(
         field_name="acknowledged_by_user",
-        queryset=get_user_queryset,
+        queryset=None,
         to_field_name="public_primary_key",
         method=ModelFieldFilterMixin.filter_model_field.__name__,
     )
     invitees_are = filters.ModelMultipleChoiceFilter(
-        queryset=get_user_queryset, to_field_name="public_primary_key", method="filter_invitees_are"
+        queryset=None, to_field_name="public_primary_key", method="filter_invitees_are"
     )
     involved_users_are = filters.ModelMultipleChoiceFilter(
-        queryset=get_user_queryset, to_field_name="public_primary_key", method="filter_by_involved_users"
+        queryset=None, to_field_name="public_primary_key", method="filter_by_involved_users"
     )
     with_resolution_note = filters.BooleanFilter(method="filter_with_resolution_note")
     mine = filters.BooleanFilter(method="filter_mine")
@@ -270,7 +295,7 @@ class AlertGroupView(
 
     pagination_class = TwentyFiveCursorPaginator
 
-    filter_backends = [SearchFilter, filters.DjangoFilterBackend]
+    filter_backends = [SearchFilter, AlertGroupFilterBackend]
     search_fields = ["public_primary_key", "inside_organization_number", "web_title_cache"]
 
     filterset_class = AlertGroupFilter