Public API: allow null escalation chain when creating routes (#1557)

# What this PR does
Allows passing `null` as a value for `escalation_chain` when creating
routes via the public API.

## Which issue(s) this PR fixes
This is needed to unblock https://github.com/grafana/oncall/pull/1555 +
creating a route without an escalation chain is possible in the web UI,
so this PR makes the public API more consistent with the web UI.

## Checklist

- [x] Tests updated
- [x] Documentation added
- [x] `CHANGELOG.md` updated

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Changed
+
+- Allow passing `null` as a value for `escalation_chain` when creating routes via the public API ([1557](https://github.com/grafana/oncall/pull/1557))
+
 ## v1.1.39 (2023-03-16)
 
 ### Added

docs/sources/oncall-api-reference/routes.md

@@ -51,7 +51,7 @@ Routes allow you to direct different alerts to different messenger channels and
 | Parameter             | Unique | Required | Description                                                                                                                                                                                                                                                                                 |
 |-----------------------| :----: |:--------:|:--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | `integration_id`      |   No   |   Yes    | Each route is assigned to a specific integration.                                                                                                                                                                                                                                           |
-| `escalation_chain_id` |   No   |   Yes    | Each route is assigned a specific escalation chain.                                                                                                                                                                                                                                         |
+| `escalation_chain_id` |   No   |   Yes    | Each route is assigned a specific escalation chain. Explicitly pass `null` to create a route without an escalation chain assigned.                                                                                                                                                          |
 | `routing_type`        |  Yes   |    No    | Routing type that can be either `jinja2` or `regex`(default value)                                                                                                                                                                                                                          |
 | `routing_regex`       |  Yes   |   Yes    | Jinja2 template or Python Regex query (use <https://regex101.com/> for debugging). OnCall chooses the route for an alert in case there is a match inside the whole alert payload.                                                                                                           |
 | `position`            |  Yes   | Optional | Route matching is performed one after another starting from position=`0`. Position=`-1` will put the route to the end of the list before `is_the_last_route`. A new route created with a position of an existing route will move the old route (and all following routes) down in the list. |

engine/apps/public_api/serializers/routes.py

@@ -155,6 +155,7 @@ class ChannelFilterSerializer(BaseChannelFilterSerializer):
     escalation_chain_id = OrganizationFilteredPrimaryKeyRelatedField(
         queryset=EscalationChain.objects,
         source="escalation_chain",
+        allow_null=True,
     )
 
     is_the_last_route = serializers.BooleanField(read_only=True, source="is_default")

engine/apps/public_api/tests/test_routes.py

@@ -162,6 +162,37 @@ def test_create_route(
     assert response.json() == expected_response
 
 
+@pytest.mark.django_db
+def test_create_route_without_escalation_chain(route_public_api_setup):
+    _, _, token, alert_receive_channel, escalation_chain, _ = route_public_api_setup
+
+    client = APIClient()
+    url = reverse("api-public:routes-list")
+
+    data = {
+        "integration_id": alert_receive_channel.public_primary_key,
+        "routing_regex": "testreg",
+        "escalation_chain_id": None,
+    }
+    response = client.post(url, format="json", HTTP_AUTHORIZATION=token, data=data)
+
+    expected_response = {
+        "id": response.data["id"],
+        "integration_id": alert_receive_channel.public_primary_key,
+        "escalation_chain_id": None,
+        "routing_type": "regex",
+        "routing_regex": data["routing_regex"],
+        "position": 0,
+        "is_the_last_route": False,
+        "slack": {"channel_id": None, "enabled": True},
+        "telegram": {"id": None, "enabled": False},
+        TEST_MESSAGING_BACKEND_FIELD: {"id": None, "enabled": False},
+    }
+
+    assert response.status_code == status.HTTP_201_CREATED
+    assert response.json() == expected_response
+
+
 @pytest.mark.django_db
 def test_invalid_route_data(
     route_public_api_setup,