SCALE transform syntax in .things file

Setup, Configuration and Use Items & Sitemaps
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  • Platform information:
    • Hardware: x64/8 GB/512 GB
    • OS: Ubuntu 22.04.5 LTS
    • Java Runtime Environment: Debian 17.0.12
    • openHAB version: 4.2.1 (official Docker version)
  • Issue of the topic: How to chain transformations in .things; especially SCALE transformation

So I have this thing (.things) configuration for my battery operated Shelly devices:

Thing mqtt:topic:shellyhtg3-543204563724 "shellyhtg3-543204563724" (mqtt:broker:mosquitto) {
    Channels:
    Type number : wifi_rssid    "RSSID"                 [ stateTopic="shellyhtg3-543204563724/status/wifi", transformationPattern="JSONPATH:$.rssi"]
}

This perfectly delivers the rssi of the wifi signal, e.g. “-67”.
The OH Shelly-binding however gives positive integer values like e.g. “2” for wireless signal strength which is nicely rendered in the UI when using the <qualityofservice> icon.

So, I stumbled upon the SCALE transformation which looked that it can transform the signal strength dB-values to those numbers known from the Shelly binding.

I now have a db.scale file:

]-55..]=4
]-65..-55]=3
]-75..-65]=2
[..-75]=1

I got it to transform my mqtt-thing values in the .items config:

Number                  HT302_WifiSignalMQTT            "SignalstÀrke"                  <qualityofservice>  (EG, GR_Kitchen)            {channel="mqtt:topic:shellyhtg3-543204563724:wifi_rssid" [profile="transform:SCALE", function="db.scale"]}

However, I thought that using the ∩ (intersection) symbol, I could somehow chain the JSONPATH and SCALE transformation already in the thing instead of the display state in the item as I am doing now.
Unfortunately, the following doesn’t work:

Type number : wifi_rssid    "RSSID"                 [ stateTopic="shellyhtg3-543204563724/status/wifi", transformationPattern="JSONPATH:$.rss∩SCALE(db.scale):%s"]

The documentation on the MQTT things shows a similar combination of transformation in the last example (above the “Limitations” section), but I can’t figure out, how the syntax for SCALE would be or if SCALE is supported in the thing transformation at all.

Any help is appreciated.

Best regards

Marco

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Please try

Type number : wifi_rssid    "RSSID"                 [ stateTopic="shellyhtg3-543204563724/status/wifi", transformationPattern="JSONPATH:$.rss∩SCALE:db.scale"]
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Hi @Udo_Hartmann,
thanks, that was the solution. I’m still struggling with slightly different syntaxes that are being used in various places.

Best regards
Marco

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