Why does this SIMPLE Kibana script not work?

[gyannea]

Following right from the documentation:

hubEvent.rssi is a String like '-98 dBm'
I want to return a number

 def rssi = doc['hubEvent.rssi.keyword'].value;
if (rssi != null)
{
    int lastdBIndex = rssi.lastIndexOf('dB');
    return Double.parseDouble(rssi.substring(lastdBIndex));
}
return null;

Following documentation here almost line for line

Using Painless in Kibana scripted fields

Yet I get an error as follows

Error executing Painless script: ' def rssi = doc['hubEvent.rssi.keyword'].value; ...'.
org.elasticsearch.index.fielddata.ScriptDocValues$Strings.get(ScriptDocValues.java:494)
org.elasticsearch.index.fielddata.ScriptDocValues$Strings.getValue(ScriptDocValues.java:508)
rssi = doc['hubEvent.rssi.keyword'].value;
                                   ^---- HERE

I have played around with all kinds of combos (String instead of 'def' which I don't understand but its in the docs with no explanation). In total frustration I just returned a string instead of a number where I just did

return doc['hubEvent.rssi.keyword'].value;

Worked like a charm. !!!???? !!! WTF???

I was hoping today's (March 7 Elastithon would provide some help). But it was all promotion. Kibana became a lot more difficult to work with after updating from 7.1.1 to 7.10.2. A lot more clicking to do the same things. Frustrating when you have issues like this to retry again and again.

[gyannea]

Basically the answer is that the documentation is wrong and inconsistent, and the error messages generated by Kibana don't help. There was one attempt where I got a different error message that did help as it clued me in to the use of .size() for the non-existence check for strings and numbers. The documentation had a different check for strings (doesn't work).

This script is what worked:

if (doc['hubEvent.rssi.keyword'].size() > 0)
{
    String rssi = doc['hubEvent.rssi.keyword'].value;
    int dbIndex = rssi.lastIndexOf('d');
    rssi = rssi.substring(0, dbIndex);
    return Double.parseDouble(rssi);
}
return null;