Message formatting from JSON to Object/Array

General
1

Warning this isn't going to be pretty, I pretty sure non of my terminology will be correct. Don't think it needs saying but I don't really know what I'm doing.

I'm trying to convert a message from JSON to Object/Array? notation. Thought I got close but in the example linked I'm getting and extra "object/array child".

What I need:
0: > "measurement" : "testDevice"

What I'm getting:
0: > "measurement" : > "payload" : "testDevice"

Think my lack of terminaloly if failing me here, as Google might be more help if I new how to search the problem. :frowning:

Function code:

var msg1 = {payload:msg.payload.device};
var msg2 = {payload:msg.payload.data};

msg.payload = [
    {
        measurement: msg1,
        fields: {
            data: msg2
        }
    }
];
return msg;

Node Example:

[{"id":"f3abc4cf.bf56a8","type":"inject","z":"ec5aafa2.fcafb","name":"JSON String","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"device\":\"testDevice\",\"data\":\"testData\"}","payloadType":"json","x":790,"y":420,"wires":[["d9595840.361e38"]]},{"id":"d9595840.361e38","type":"function","z":"ec5aafa2.fcafb","name":"","func":"var msg1 = {payload:msg.payload.device};\nvar msg2 = {payload:msg.payload.data};\n\nmsg.payload = [\n    {\n        measurement: msg1,\n        fields: {\n            data: msg2\n        }\n    }\n];\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":980,"y":420,"wires":[["23112814.a43278"]]},{"id":"23112814.a43278","type":"debug","z":"ec5aafa2.fcafb","name":"Incorrect","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":1160,"y":420,"wires":[]},{"id":"281bada5.7f2602","type":"function","z":"ec5aafa2.fcafb","name":"","func":"msg.payload = [\n    {\n        measurement: \"testDevice\",\n        fields: {\n            data: \"testData\",\n        },\n    }\n];\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":980,"y":380,"wires":[["1289e8ef.e81067"]]},{"id":"4def3a8e.7d8624","type":"inject","z":"ec5aafa2.fcafb","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":780,"y":380,"wires":[["281bada5.7f2602"]]},{"id":"1289e8ef.e81067","type":"debug","z":"ec5aafa2.fcafb","name":"Correct","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":1160,"y":380,"wires":[]}]
1 Like
2

I am afraid your notation is a bit awry as you suggest. The output of the lower inject node is not json, it is a javascript object. The string you have entered into the inject node config is json, but you have told it to convert that to a javascript object.

You are nearly there, you are overthinking the problem. All you need it something like

msg.payload = [
    {
        measurement: msg.payload.device,
        fields: {
            data: msg.payload.data
        }
    }
];
return msg;

or if you want to pick up the values to a local variable first then

let measurement = msg.payload.device
let data = msg.payload.data

msg.payload = [
    {
        measurement: measurement,
        fields: {
            data: data
        }
    }
];
return msg;

Have a read of this page from the docs, it should help you to understand messages. Working with messages : Node-RED

1 Like
3

Just to provide another way to accomplish the same thing: I like using a JSONata expression inside a change node for cases like this. For your sample data, the J: expression would be:

[
    {
        measurement: payload.device,
        fields: {
            data: payload.data
        }
    }
]

JSONata uses a less familiar "lambda" expression syntax, which looks similar to JS -- and so most people prefer to avoid it and use plain javascript. But it provides a rather powerful syntax to transform JS object structures... similar in design to the way XSLT uses XPath to transform XML structures.

FWIW, I think it's a nice tool to have at hand -- and while it's easy to learn, it does take a while to master. If you want to learn more, you can try it out at try.jsonata.org

2 Likes
4

Thank you guys, just the input I needed.

@Colin, sorry I wasn't very articulate on my requirements. I wanted the JSON inject to output at the debug as a javascript object, to match the example provided on top. I fear I'm just typing in circles now. :rofl: Anyway, thanks again!

@shrickus, thanks, you might have lost me a bit but I'll do some homework.

5

I think your code is confusing people. If you require just an array containing a object with one key value pair, it would be more like below.

msg.payload = [
    {
    measurement: msg.payload.data
    }
];
6

Would that not be "Invalid JSONata expression, Key must evaluate to a string" as your keys would need to be in quotes.

7

Maybe back round would have been a better place to start.

I am taking a callback and sending it to InfluxDB. The callback arrives as a JSON (string/object - please correct me to which one) as in my example of the JSON inject node. As with the example from the InfluxDB batch node (node-red-contrib-influxdb (node) - Node-RED), it needs to be a javascript object.

My understanding of all this is very limited so apologies for the confusion but thank you for the help.

8

Of course, you are right... too big a hurry.

And now I see he wants a simpler output format anyway.

9

Feed it into a debug node and if it says it is a string it is a string. If it is in JSON format then it is a JSON string, also known as a JSON object. If the debug node shows it as an object then it is a javascript object.

Whichever it is, show us what it looks like so we can understand what you are starting with.

10

Looks like you are looking for JSON.parse(string) with string is a json-structure.
I have no access to my node-red instance right now to test your code.

11

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