I want to update documents, and add  some of the fields to an array, e.g. with documents from json like

{ "name": "bill, "hobbies": "a" , "id": 1}

if I run logstash on more data like

{ "name": "ted, "hobbies": "a" , "id": 2}
{ "name": "bill, "hobbies": "c" , "id": 1}

I'd like to end up with ted having ["a"] as hobbies, and bill with ["a", "c"].

Looking at https://discuss.elastic.co/t/creating-updating-array-of-objects-in-elasticsearch-logstash-output/59139 and https://discuss.elastic.co/t/add-new-values-in-the-nested-field-logstash-to-elasticsearch/62253 I've tried adding a script to the output plugin for elasticsearch

        script => '
          if (ctx._source.hobbies != null) {
          } else {
              ctx._source.hobbies = [params.event.get("hobbies")]

This fails on the update because the field is a string;

 "reason"=>"Unable to find dynamic method [add] with [1] arguments for class [java.lang.String]."}

How do I persuade it to be an array?
Is this possible? Is there another option?


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