Search for data in msg. payload based on values within payload. Is this possible?

General
1

Hi,

I'm using node red for a while now as rule engine for my home automation system. Now I've stumbled over a problem, I cannot solve by myself. Every idea how to solve is highly welcome :slight_smile:

The problem is hard to explain, because I'm not a native English speaker, but I try to...

I try to get information about my heating system with a Rest-API call and want to use the received information (as object) in a flow. This works so far but...

When requesting e.g. "outside temperature" from my heating system the received msg-object looks like this (shortened):

{
    "_msgid": "b4d8608.3e462a",
    "payload": {
        "properties": {
            "value": {
                "value": 8.2,
                "type": "number"
            },
            "status": {
                "value": "connected",
                "type": "string"
            },
            "unit": {
                "value": "celsius",
                "type": "string"
            }
        },
        "path": "properties.value.value"
    },
    "statusCode": 200
}

So the value of msg.payload.path tells me where to find the value I want to extract. In this example it is at msg.payload.properties.vale.value

Another example when asking the REST-API e.g. for the "outside temperature" is:

{
    "_msgid": "5f35fd23.ced484",
    "payload": {
        "properties": {
            "active": {
                "value": true,
                "type": "boolean"
            },
            "demand": {
                "value": "unknown",
                "type": "string"
            },
            "temperature": {
                "value": 22,
                "type": "number"
            }
        },
        "path": "properties.temperature.value"
    },
    "statusCode": 200
}

Here the value of msg.payload.path tells me that the value I want to extract is at msg.payload.properties.temperature.value

Is it possible, maybe with a change node and JSONata, to first get the value of msg.path and use this value as path to the value I want to extract??? How???

I played a bit with JSONata but with our success :frowning:

Regards,
Michael

2

Welcome to the forum @NoMercyMike.

In a function node this is one way to do it

let value = eval(`msg.payload.${msg.payload.path}`)

but I don't like using eval. I suspect there is a better way using something like
msg.payload[msg.payload.path]
but that doesn't work.

3

Yes the same way as in function node with $eval
e.g.

[{"id":"932dafe1.4093b8","type":"inject","z":"8d22ae29.7df6d","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"properties\":{\"value\":{\"value\":8.2,\"type\":\"number\"},\"status\":{\"value\":\"connected\",\"type\":\"string\"},\"unit\":{\"value\":\"celsius\",\"type\":\"string\"}},\"path\":\"properties.value.value\"}","payloadType":"json","x":120,"y":2500,"wires":[["e8ea619b.7e60a","f98b95cf.92cb"]]},{"id":"e8ea619b.7e60a","type":"change","z":"8d22ae29.7df6d","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$eval(\"payload.\" & payload.path)","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":330,"y":2500,"wires":[["f98b95cf.92cb"]]},{"id":"f98b95cf.92cb","type":"debug","z":"8d22ae29.7df6d","name":"Na Template","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":620,"y":2300,"wires":[]}]
4

The way I would do it is to use the JSONata $eval(...) function inside a change node, with this expression:

payload.$eval(path)

I tried the $lookup(...) function, but that only appears to search 1 level, not the 3 listed in the 'path' string...

1 Like
5

Without eval

var path = msg.payload.path.split(".")
while(path.length){
    msg.payload = msg.payload[path.shift()]
}
return msg;
6

I like that way. Eval is dangerous as if you get something unexpected, or even malicious, in path it will execute it.

7

It is just a quick and dirty example. If path is incorrect it fails pretty hard. So needs improvements for real usage.

8

You can use the builtin helper function RED.util.getMessageProperty ...

getMessageProperty(msg,expr)
msg is the object to "get" a value from
expr is the path to the item

so instead of passing msg in, pass msg.payload in.

FUNCTION CODE...

msg.value = RED.util.getMessageProperty(msg.payload, msg.payload.path);
return msg;

Demo...

[{"id":"2a6df415.f905fc","type":"inject","z":"1ce23005.82287","name":"fake it","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{     \"properties\": {         \"active\": {             \"value\": true,             \"type\": \"boolean\"         },         \"demand\": {             \"value\": \"unknown\",             \"type\": \"string\"         },         \"temperature\": {             \"value\": 22,             \"type\": \"number\"         }     },     \"path\": \"properties.temperature.value\" }","payloadType":"json","x":380,"y":780,"wires":[["158d361d.bd639a"]]},{"id":"158d361d.bd639a","type":"function","z":"1ce23005.82287","name":"","func":"debugger\nmsg.value = RED.util.getMessageProperty(msg.payload, msg.payload.path);\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":530,"y":780,"wires":[["e5e22824.4b89c8"]]},{"id":"e5e22824.4b89c8","type":"debug","z":"1ce23005.82287","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":690,"y":780,"wires":[]}]
2 Likes
9

Many thanks @all :slight_smile:

At least the solution from Steve-Mci worked for me. Great!!!

10

True, I always try to avoid using eval in Javascript... but I'm less worried about it inside a JSONata expression, where the data is all sand-boxed and there is no access to the file system.

11

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