Filter to get only some data from msg.payload: string

[pwgrau]

Hello, I have this data via mqtt and I want to take it to an influxdb to graph it in a grafana, the values ​​that arrive in the graph are this whole chain, and I just need to get temperature and humidity data in number:

{"Time":"2020-06-11T13:36:11","SI7021":{"Temperature":15.9,"Humidity":74.5,"DewPoint":11.4},"TempUnit":"C"}

I am a novice in node-red I would appreciate that you explain with what specific nodes I could make these filters.

[Colin]

Have a read in the node red docs at the section Working with Messages. That should help a lot.
[Edit] also, if the example you posted is a string then set the MQTT node to output Parsed JSON. It will convert the JSON string to a javascript object which will make it much easier.

[mtoko]

The data is a JSON Object and you can extract the temperature and humidity values using a change node. See the example flow. It is possible that the message from the MQTT node is a JSON string and you will need to add a JSON node to convert the output of the MQTT node to a JSON Object

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    {
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        "type": "tab",
        "label": "Flow 7",
        "disabled": false,
        "info": ""
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    {
        "id": "fc5901ca.92592",
        "type": "inject",
        "z": "6ff4951.acf306c",
        "name": "",
        "topic": "",
        "payload": "{\"Time\":\"2020-06-11T13:36:11\",\"SI7021\":{\"Temperature\":15.9,\"Humidity\":74.5,\"DewPoint\":11.4},\"TempUnit\":\"C\"}",
        "payloadType": "json",
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    {
        "id": "1d4f00f9.b6b25f",
        "type": "change",
        "z": "6ff4951.acf306c",
        "name": "",
        "rules": [
            {
                "t": "set",
                "p": "payload",
                "pt": "msg",
                "to": "payload.SI7021.Temperature",
                "tot": "msg"
            }
        ],
        "action": "",
        "property": "",
        "from": "",
        "to": "",
        "reg": false,
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    },
    {
        "id": "d316e8b9.a21aa8",
        "type": "change",
        "z": "6ff4951.acf306c",
        "name": "",
        "rules": [
            {
                "t": "set",
                "p": "payload",
                "pt": "msg",
                "to": "payload.SI7021.Humidity",
                "tot": "msg"
            }
        ],
        "action": "",
        "property": "",
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        "to": "",
        "reg": false,
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        "wires": [
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        "complete": "false",
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    }
]

[Colin]

@mtoko

you will need to add a JSON node

The MQTT node has this capability built in, just tell it to parse the JSON string and convert it to a javascript object (note, a javascript object, it is not correct to call it a JSON object, JSON is always a string).

[mtoko]

Thanks Colin, always learning!

[pwgrau]

I passed it to json I have it as an object and I could remove some data with the change node, I think I have to improve the filter so as not to display and have the pure data I need such as temperature and humidity

[mtoko]

[pwgrau]

Thank you all very much, it worked that way