Script access to nested field

[GR8 (Gabriel)]

Hello Elastic folks,

I need to run a script that gets data from index_a to index_b with some basic ETL.

This works well except I can't figure out the syntax for accessing a nested field like:

"transitions": {
  "properties": {
    "channel": {
      "type": "keyword"
    },
    "created_at": {
      "format": "epoch_millis",
      "type": "date"
    },

My _reindex script looks like this:

if (ctx._source.transitions != null && ctx._source.transitions.created_at != null) {
  ctx._source.transitions.created_at =
    ctx._source.transitions.created_at / 1000000;
}

…and returns the error

     "script_stack" : [
      "if(ctx._source.transitions!=null&&ctx._source.transitions.created_at!=null){",
      "                                                         ^---- HERE"
    ],
…
    "caused_by" : {
      "type" : "illegal_argument_exception",
      "reason" : "Illegal list shortcut value [created_at]."

It feels like I'm missing something obvious.

Thanks for your help!