What's the preferred way to update part of a nested document

[viebel (Yehonathan Sharvit)]

My document has a nested structure like this:

PUT /customer/_doc/1
{
  "name": "hello",
  "body": {
    "module": "aa",
    "config": "bb"
  }
}

Now, I'd like to update body field to be:

{
    "module": "cc",
    "args": "dd"
}

Pay attention I need to:

1. update the value associated to module
2. remove the config field
3. add an args field

I tried to use update API

POST /customer/_update/1
{
  "doc": {
    "body":     {
        "module": "cc",
        "args": "dd"
    }
  }
}

It almost works, but the problem is that the config field is not removed

GET /customer/_doc/1

{
 ...
  "_source" : {
    "name" : "hello",
    "body" : {
      "module" : "cc",
      "config" : "bb",
      "args" : "dd"
    }
  }

}

Is the exact merging algorithm used by update API documented somewhere?

Is there a way to achieve this without writing a script?
I don't want to get the document, do the merge and ind it again ( I prefer to use the update API to avoid roundtrips and benefit from auto retry mechanism)