Problem understanding MQTT value templates

[maxdepoli]

Hi all.
I’m exploring MQTT sensor integrations, and I’m stumped in acquiring data from json payload.

I have this esp32 device that simply sends out this json

{
    "ID": "testgen",
    "SigStr": "-49",
    "uptime": "124610"
}

on MQTT_Test/testgen/status topic; broker sees it with no problem and I can parse it from NodeRed.

so I try to add an integration to show the “uptime” value.

First try, I tried to use MQTT discover:

 char jsonString[2048];
 char discoveryTopic[50];
 JsonDocument dataPacket;

  MQTT.setServer(mqttBroker, 1883);
  MQTT.connect(hostname,MQTT_User,MQTT_PW);
  while (millis()-BeginConnect<timeout && !MQTT.connected()) {};
  if (!MQTT.connected()) {
    ESP_LOGE("MQTTConnect","Connection attempt timed out");
    return false;
  } else {
    dataPacket["dev"]["ids"]="testgen";
    dataPacket["dev"]["name"]="Uptime test";
    dataPacket["o"]["name"]="MQTT uptime test";

    dataPacket["cmps"]["testgen_T"]["p"]="sensor";
    dataPacket["cmps"]["testgen_T"]["unit_of_measurement"]="ms";
    dataPacket["cmps"]["testgen_T"]["value_template"]="{{ value_json.uptime }}";
    dataPacket["cmps"]["testgen_T"]["unique_id"]="testgen_T";

    dataPacket["state_topic"]="MQTT_Test/testgen/status";
    dataPacket["qos"]= "0";
    serializeJson(dataPacket,jsonString);
    
    sprintf(discoveryTopic,"homeassistant/device/testgen/config");
    MQTT.publish(discoveryTopic,jsonString);

this built a discover json

{
  "dev": {
    "ids": "testgen",
    "name": "Uptime test"
  },
  "o": {
    "name": "MQTT uptime test"
  },
  "cmps": {
    "testgen_T": {
      "p": "sensor",
      "unit_of_measurement": "ms",
      "value_template": "{{ value_json.uptime }}",
      "unique_id": "testgen_T"
    }
  },
  "state_topic": "MQTT_Test/testgen/status",
  "qos": "0"
}

but apparently it could not be published; tried to push it through with the MQTT broker, but apparently it did not like the “{{ value_json.uptime }}” part.

So I tried adding manually a MQTT device, but still, did not like “value_json” in the template.

What am I missing?

[Spiro]

@maxdepoli

I try to add an integration to show the “uptime” value.

If you mean a sensor then it would be something like this in yaml

 mqtt:
   sensor:
     - name: "Testuptime"
       device_class: "uptime"
       state_topic: "MQTT_Test/testgen/status"
       value_template: "{{value_json.uptime }}"
       unit_of_measurement: 's'
       unique_id: test_uptime 

MQTT Sensor

[maxdepoli]

Yes, did that, but I get… a sensor that gets no value ever.

.

Also, I don’t understand why I can’t get a working MQTT discovery packet, I copied the routine almost verbatim from several tutorials I found around…

[Spiro]

 mqtt:
   sensor:
     - name: "Testuptime"
       state_topic: "MQTT_Test/testgen/status"
       value_template: "{{value_json.uptime }}"
       unit_of_measurement: 's'
       unique_id: test_uptime 

Remove the device class. I looked again “uptime”: “124610”. The “” round the number I think means its a string. It’s better to view in a card like entity rather than a graph.

[jhol-byte]

also have a look into the HA log, if the payload cannot be rendered as valid JSON, value_json will be undefined

[123 (Taras)]

@maxdepoli

apparently it did not like the “{{ value_json.uptime }}” part.

That’s because Home Assistant attempted to evaluate the template before publishing the payload. You want it to publish the payload without first evaluating the template.

Here is how I do it. The following example worked for me. Notice the use of {%raw%} and {%endraw%} tags surrounding the template. It ensures the template is not evaluated before publishing the payload.

{
  "dev": {
    "ids": "testgen",
    "name": "Uptime test"
  },
  "o": {
    "name": "MQTT uptime test"
  },
  "cmps": {
    "testgen_T": {
      "p": "sensor",
      "unit_of_measurement": "ms",
      "value_template": {%raw%}"{{value_json.uptime}}"{%endraw%},
      "unique_id": "testgen_T"
    }
  },
  "state_topic": "MQTT_Test/testgen/status",
  "qos": "0"
}

I then published a value of {"uptime": 100} to MQTT_Test/testgen/status and confirmed the sensor reports 100.