Convert two repeated values in array into a string - elasticsearch

Question

I have some old documents where a field has an array of two vales repeated, something like this:
"task" : [
"first_task",
"first_task"
],
I'm trying to convert this array into a string because it's the same value. I've seen the following script: Convert array with 2 equal values to single value but in my case, this problem can't be fixed through logstash because it happens just with old documents stored.
I was thinking to do something like this:
POST _ingest/pipeline/_simulate
{
"pipeline": {
"processors": [
{
"script": {
"description": "Change task field from array to first element of this one",
"lang": "painless",
"source": """
if (ctx['task'][0] == ctx['task'][1]) {
ctx['task'] = ctx['task'][0];
}
"""
}
}
]
},
"docs": [
{
"_index" : "tasks",
"_type" : "_doc",
"_id" : "1",
"_score" : 1.0,
"_source" : {
"#timestamp" : "2022-05-03T07:33:44.652Z",
"task" : ["first_task", "first_task"]
}
}
]
}
The result document is the following:
{
"docs" : [
{
"doc" : {
"_index" : "tasks",
"_type" : "_doc",
"_id" : "1",
"_source" : {
"#timestamp" : "2022-05-03T07:33:44.652Z",
"task" : "first_task"
},
"_ingest" : {
"timestamp" : "2022-05-11T09:08:48.150815183Z"
}
}
}
]
}
We can see the task field is reassigned and we have the first element of the array as a value.
Is there a way to manipulate actual data from Elasticsearch and convert all the documents with this characteristic using DSL queries?
Thanks.

Answer 1

You can achieve this with _update_by_query endpoint. Here is an example:
POST tasks/_update_by_query
{
"script": {
"source": """
if (ctx._source['task'][0] == ctx._source['task'][1]) {
ctx._source['task'] = ctx._source['task'][0];
}
""",
"lang": "painless"
},
"query": {
"match_all": {}
}
}
You can remove the match_all query if you want to update all documents or you can filter documents by chaning the conditions in the query.
Keep in mind that running a script to update all documents in the index may cause some performance issues while the update process is running.