Extract values from array inside json

[koder29406]

Hello!
I'm trying to convert a JOLT spec to JLST spec. The following worked with JOLT, but I don't get how to get jlst to do the same thing.
Input:

{
  "sourceId": "SN98640:0",
  "referenceTime": "2024-04-01T23:00:00.000Z",
  "observations": [
    {
      "elementId": "air_temperature",
      "value": -9.4,
      "exposureCategory": "2"
    },
    {
      "elementId": "wind_speed",
      "value": 8.3,
      "exposureCategory": "2"
    }
  ]
}

Desired output, which I get from JOLT:

{
  "stasjonsid" : "SN98640:0",
  "tid" : "2024-04-01T23:00:00.000Z",
  "temperatur" : -9.4,
  "vindstyrke" : 8.3
}

Working JOLT spec:

[
  {
    "operation": "shift",
    "spec": {
      "sourceId": "stasjonsid",
      "referenceTime": "tid",
      "observations": {
        "*": {
          "value": "@(1,elementId)"
        }
      }
    }
  },
  {
    "operation": "shift",
    "spec": {
      "air_temperature": "temperatur",
      "wind_speed": "vindstyrke",
      "referenceTime": "tid",
      "*": {
        "@": "&"
      }
    }
  }
]

Any suggestions how to fix this?

[larsga]

The way to write a transform like this in JSLT is to start with the output you want. Use that as the starting point. Then go through the values and replace them with a JSLT expression that will find the correct value in the input. Use the playground to try it out as you go. If you keep the tutorial open you should be able to do solve the first two pretty much immediately.

[koder29406]

Thanks, I read the examples earlier and got this far:

{
  "stasjonsid" : .sourceId,
  "tid" : .referenceTime,
  "observations" : {for (.observations)
    .elementId : .value
  }
}

{
  "stasjonsid" : "SN98640:0",
  "tid" : "2024-04-01T23:00:00.000Z",
  "observations" : {
    "air_temperature" : -9.4,
    "wind_speed" : 8.3
  }
}

I can't get the temperatue and wind to be on the root level instead of inside observations.

It feels like this spec should work, but I get a parse error.

{
  "stasjonsid" : .sourceId,
  "tid" : .referenceTime,
  { for ( .observations) .elementId : .value }
}

[larsga]

There's a few different ways you can deal with observations. The big question is: is the order reliable? It probably isn't.

You need to first find the observation that has "air_temperature", and only afterwards can you get the value. Try this:

[for (.observations) .value if (.elementId == "air_temperature")]

It will get you an array. Now you need to pick out the first element from it. Unfortunately, you can't just tack on a [0].

let air_temperatures = [for (.observations) .value if (.elementId == "air_temperature")]
$air_temperatures[0]

will do it.

Put the two lets at the top and you'll have it.

[koder29406]

Thanks, this worked!

let air_temperatures = [for (.observations) .value if (.elementId == "air_temperature")]
let vind = [for (.observations) .value if (.elementId == "wind_speed")]

{
  "stasjonsid" : .sourceId,
  "tid" : .referenceTime,
  "temperatur" : $air_temperatures[0],
  "vind" : $vind[0]
}

What if i had more measurements than temperature and wind. What would be a good generic way to get for example all elementIds and values?

[larsga]

You kind of did get all of them already with this:

  "observations" : {for (.observations)
    .elementId : .value
  }

What you could do is make one object with the first two fields, then one that has all observations, and + them together into a single object:

{
  "stasjonsid" : .sourceId,
  "tid" : .referenceTime
 } + {
  for (.observations)
    .elementId : .value
}

Of course, this gives you the last part in the wrong vocabulary. You could define a function that translates the elementId into the right term. One way to do that is just with if statements, but you could also do let mapping = {} and then use get-key() to get the mapping.

[koder29406]

Thank you for your help. I'll take a look at that later!