Access Nested Objects using Painless for Elasticsearch/Opensearch Alerting to Create Trigger Condition - opensearch

Question

I tried creating Trigger condition in Monitor in Opensearch.I was trying to extract fields present in _source which is giving error "reason" : "Illegal list shortcut value [_source].".
DSL Extraction Query-
{
"size": 1,
"sort": {"Date" : "desc" },
"query": {
"bool": {
"filter": [
{
"range": {
"Date": {
"from": "{{period_end}}||-3d",
"to": "{{period_end}}",
"include_lower": true,
"include_upper": true,
"format": "epoch_millis",
"boost": 1
}
}
}
],
"adjust_pure_negative": true,
"boost": 1
}
},
"aggregations": {
"metric": {
"avg": {
"field": "difference"
}
}
}
}
Output-
{
"_shards": {
"total": 4,
"failed": 0,
"successful": 4,
"skipped": 2
},
"hits": {
"hits": [
{
"_index": "reliance_cal_spread",
"_source": {
"symbol": "RELIANCE",
"std_dev": 2.4,
"upper_range": 10.3,
"current_month_close": 2536.3,
"mean": 7.9,
"weekday": "Tuesday",
"difference": 3,
"Date": "2022-06-28T00:00:00",
"next_month_close": 2539.3,
"lower_range": 5.5
},
"_id": "E2-cqoEB0vMP3J-sGCIB",
"sort": [
1656374400000
],
"_score": null
}
],
"total": {
"value": 2,
"relation": "eq"
},
"max_score": null
},
"took": 8,
"timed_out": false,
"aggregations": {
"metric": {
"value": 2.799999952316284
}
}
}
Trigger Condition-
return ctx.results[0].aggregations.metric.value == null ? false :ctx.results[0].hits.hits._source.difference > 6
Error-
{
"type" : "script_exception",
"reason" : "runtime error",
"script_stack" : [
"return ctx.results[0].aggregations.metric.value == null ? false :ctx.results[0].hits.hits._source.difference > 6",
" ^---- HERE"
],
"script" : "return ctx.results[0].aggregations.metric.value == null ? false :ctx.results[0].hits.hits._source.difference > 6",
"lang" : "painless",
"position" : {
"offset" : 89,
"start" : 0,
"end" : 112
},
"caused_by" : {
"type" : "illegal_argument_exception",
"reason" : "Illegal list shortcut value [_source]."
}
}
While I am able to extract other conditions such as-aggregations.metric.value and hits.total.value.
Trigger Condition
return ctx.results[0].aggregations.metric.value == null ? false :ctx.results[0].aggregations.metric.value < 6
Trigger Condition Response-
True