JSONata object creation (with $each?)

General
1

Hi everyone,
I'm new to JSONata and I'm struggling to get this right.
I have some data that I need to transform for graphing in node-red. I've had a look through the documentation and examples, and I'm almost there but I can't quite get object creation working in this context.

Here's a data structure example to illustrate my issue: https://try.jsonata.org/vGGPlt_N7
However for a line chart in node-red dashboards, the data format needs to be:

"data": [
      {"x": 1, "y": 13.8},
      {"x": 2, "y": 14.1},
      {"x": 3, "y": 14.4},
      {"x": 4, "y": 14.5},
      {"x": 5, "y": 14.4}
    ],

The x values are not fixed, I may choose to put something else there (eg. a timestamp), but for now they are just placeholders to work out the JSONata.

I can't seem to find a way to do this when using $each, as in the above link. However I'm not even really sure I should be using $each - it seems that this takes all the results from each object and flattens them into an array. I actually need to form an object for each measurement so maybe this is counterproductive.

I have also tried something like "data":temp, in JSONata, which produces a similar array of values, but I would need to transform these into objects as well.
I also need to perform division/string manipulation on the values, and I can't do something like "data":temp / 100,. I think this is because it would be trying to divide the entire array by 100, and hence doesn't work, whereas I want to actually do an element-wise division of each value. The function definition with $each allows me to do this.

Maybe there is a very simple way of accomplishing this as well as the object issue - it feels like a fairly simple problem but none of the examples or things I've tried so far have really succeeded. Please let me know if anyone has any advice! Thanks.

2

Welcome...

try this thread, it may lead to what you want..

3

Thanks a lot for this.

"data": [[$.{
        "x": id,
        "y": temp
    }]],

This was enough to match the structure I want. I feel like I tried so many permutations of this sort of syntax but could never get it to parse properly.. I think it's the extra square brackets that I wasn't trying. Thanks again!!

1 Like
4

The issue you have is where is this array , is it in payload? If so then you did not set the input array correct, it needs to be in a object with the property payload.

like this https://try.jsonata.org/RfHWFmZei

then you would end up with a expression

[
   {
       "series": ["Temp"],
       "data": [[$$.payload.{"x": $.id*1000, "y": $.temp/100}]],
       "labels":["Temp"]
   }
]

Multiplied by 1000 so the times are 1 second apart, edit to fit. A square brackets around an expression will force a single result to an array, so is ignored if the result is an array, that is why you need the extra array bracket.

5

Yes sorry, I forgot to mention that I was testing on the jsonata.org website and I'm adding in payload on the actual node-red config. I've just spend a little bit adding some rounding and other adjustments to your code, and I'm now getting a proper line graph correctly in the dashboard, so it works!

This syntax is doing my head in. Lots more learning for me.

6

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