Copy part of Elasticsearch field to new one

Is there a way to copy part of a field into a new one in ES when indexing :

I'd like to have something like this :

{
    "key": "123",
    "type": "novel",
    "description": "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book.",
    "preview": "Lorem Ipsum is simply dummy..."
}

Basically "preview" is a copy of "description" but only a few first characters.
I checked the copy_to method in Elasticsearch but there are no option to do this.

This can be achieved with an ingest pipeline and a script processor

POST _ingest/pipeline/_simulate
{
  "pipeline": {
    "processors": [
      {
        "script": {
          "description": "Summarize the description field",
          "lang": "painless",
          "source": "ctx.preview = ctx.description.substring(0, 20) + '...'"
        }
      }
    ]
  },
  "docs": [
    {
      "_source": {
        "description": "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book."
      }
    }
  ]
}

Response:

{
  "docs" : [
    {
      "doc" : {
        "_source" : {
          "preview" : "Lorem Ipsum is simpl...",
          "description" : "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book."
        }
      }
    }
  ]
}

As an alternative instead of taking a fixed amount of characters, you could also take a fixed amount of words. Easy to change.