JEXL. Expression evaluation on query results

[fgalan]

Is your feature request related to a problem / use case? Please describe.

Use JEXL in queries so an expression can be evaluated on the query results to define virtual attributes.

Describe the solution you'd like

Let's consider an operation like this:

POST /v2/op/query

{
  "entities": [
    {
      "idPattern": ".*",
      "type": "Sensor"
    }
  ],
  "attrs": [
    "temperature"
  ],
  "virtualAttrs": {
      "tempRatio": {
        "value": "${temperature/100}",
        "type": "Number"
      }
  }
}

So assuming we have the following entities:

• Sensor01 / Sensor, temperature=20
• Sensor02 / Sensor, temperature=40

The response to that operation is:

• Sensor01 / Sensor, temperature=20, tempRatio=0.02
• Sensor02 / Sensor, temperature=40, tempRatio=0.04

Describe alternatives you've considered

Alternative syntax:

  "virtualAttrs": {
      "tempRatio": {
        "value": "temperature/100",
        "type": "Number"
      }
  }

Pros: simplified sintax avoiding ${...}
Const: lack of homogenity compared with httpCustom.ngsi in subscriptions

We have REST-style query ops (GET /v2/entities or GET /v2/entities/{id}) but it would be more complicated defining virtualAttrs on them (they don't use payload in the request).

Describe why you need this feature

• To approach a new use case.
• To improve or simplify an scenario.

[mapedraza]

Probably related:
#3815