How to create auto generated ids while reindexing?

[SudevPK]

How to change _id as GUID during reindex

I have two different elastic search nodes.I need to reindex 10 indices from one node to 1 index in another node.But there is not unique id's in the 10 indices, so i need to generate _id as GUID while reindexing

I am linking my old post of same
Try

POST _reindex
{
  "source": {
    "index": ["twitter", "twitter2"]
  },
  "dest": {
    "index": "new_twitter"
  },
  "script": {
    "source": "ctx._id=null",
    "lang": "painless"
  }
}