Add entity_id to mqtt sensor templates (#50773)

* Add entity_id to mqtt sensor

* update test comment

homeassistant/components/mqtt/sensor.py

@@ -125,8 +125,11 @@ class MqttSensor(MqttEntity, SensorEntity):
 
             template = self._config.get(CONF_VALUE_TEMPLATE)
             if template is not None:
+                variables = {"entity_id": self.entity_id}
                 payload = template.async_render_with_possible_json_value(
-                    payload, self._state
+                    payload,
+                    self._state,
+                    variables=variables,
                 )
             self._state = payload
             self.async_write_ha_state()

tests/components/mqtt/test_sensor.py

@@ -640,3 +640,31 @@ async def test_entity_disabled_by_default(hass, mqtt_mock):
     await help_test_entity_disabled_by_default(
         hass, mqtt_mock, sensor.DOMAIN, DEFAULT_CONFIG
     )
+
+
+async def test_value_template_with_entity_id(hass, mqtt_mock):
+    """Test the access to attributes in value_template via the entity_id."""
+    assert await async_setup_component(
+        hass,
+        sensor.DOMAIN,
+        {
+            sensor.DOMAIN: {
+                "platform": "mqtt",
+                "name": "test",
+                "state_topic": "test-topic",
+                "unit_of_measurement": "fav unit",
+                "value_template": '\
+                {% if state_attr(entity_id, "friendly_name") == "test" %} \
+                    {{ value | int + 1 }} \
+                {% else %} \
+                    {{ value }} \
+                {% endif %}',
+            }
+        },
+    )
+    await hass.async_block_till_done()
+
+    async_fire_mqtt_message(hass, "test-topic", "100")
+    state = hass.states.get("sensor.test")
+
+    assert state.state == "101"