Fix alert not firing when the watched entity is already in the alert … (#181218)

Co-authored-by: Sebastian Messmer <2373925+smessmer@users.noreply.github.com>

homeassistant/components/alert/entity.py

@@ -72,12 +72,24 @@ class AlertEntity(Entity):
         self._ack = False
         self._cancel: Callable[[], None] | None = None
         self._send_done_message = False
+        self._watched_entity_id = watched_entity_id
         self.entity_id = f"{DOMAIN}.{entity_id}"
 
         async_track_state_change_event(
             hass, [watched_entity_id], self.watched_entity_change
         )
 
+    @override
+    async def async_added_to_hass(self) -> None:
+        """Start alerting if the watched entity is already in the alert state."""
+        await super().async_added_to_hass()
+        if (
+            (state := self.hass.states.get(self._watched_entity_id)) is not None
+            and state.state == self._alert_state
+            and not self._firing
+        ):
+            await self.begin_alerting()
+
     @property
     @override
     def state(self) -> str:

tests/components/alert/test_init.py

@@ -94,6 +94,32 @@ async def test_fire(hass: HomeAssistant, mock_notifier: list[ServiceCall]) -> No
     assert hass.states.get(ENTITY_ID).state == STATE_ON
 
 
+async def test_fire_when_already_in_alert_state(
+    hass: HomeAssistant, mock_notifier: list[ServiceCall]
+) -> None:
+    """Test the alert fires if the watched entity is already in the alert state.
+
+    This is the case after a restart when the watched entity's state is
+    restored before the alert subscribes to state changes.
+    """
+    hass.states.async_set("sensor.test", STATE_ON)
+    assert await async_setup_component(hass, DOMAIN, TEST_CONFIG)
+    await hass.async_block_till_done()
+    assert hass.states.get(ENTITY_ID).state == STATE_ON
+    assert len(mock_notifier) == 1
+
+
+async def test_idle_when_already_not_in_alert_state(
+    hass: HomeAssistant, mock_notifier: list[ServiceCall]
+) -> None:
+    """Test the alert stays idle if the watched entity is not in the alert state."""
+    hass.states.async_set("sensor.test", STATE_OFF)
+    assert await async_setup_component(hass, DOMAIN, TEST_CONFIG)
+    await hass.async_block_till_done()
+    assert hass.states.get(ENTITY_ID).state == STATE_IDLE
+    assert len(mock_notifier) == 0
+
+
 async def test_silence(hass: HomeAssistant, mock_notifier: list[ServiceCall]) -> None:
     """Test silencing the alert."""
     assert await async_setup_component(hass, DOMAIN, TEST_CONFIG)