Update by query should support short script form and `doc`

[clintongormley]

The script parameter in update-by-query requires a full script object, instead of allowing the simple script form:

"script": "ctx._source.foo=5"

We should be using the same script parser we use every else, which would take care of this automatically.

Also, any reason why we don't support partial doc updates with the doc parameter`?

[rjernst]

What do you mean by partial updates? doc is doc values, ie already indexed values.

[clintongormley]

No, I mean the doc parameter which exists in the update query, as in:

POST index/type/id/_update
{
  "doc": {
    "status": "published"
  }
}

This request would merge the doc object with the _source. It's currently not supported by update-by-query

[rjernst]

Ok, thanks for the clarification. So this would happen outside of the script itself.

[pozhidaevak]

Hi,
I want to make this issue my first contribution to the project. Has anyone started work on this issue? Is it ok if I start working on it? It will probably take some time for me to understand the issue and familiarise myself with the code.

[nik9000]

This is two issues:

1. Update-by-query doesn't support doc style updates like those mentioned here.
2. Update-by-query doesn't support a short form for specifying the script.

The reasoning behind the first point is that we wanted to keep update-by-query simple. We can do it though. I don't think solving the first point would be a good first contribution, but it is a thing we can do.

The second point is fine for a first contribution because it is just extra parsing without any additional changes. I don't know why it doesn't work now but to fix it I'd hunt down how we do parsing with _update and see if we can use that for _update_by_query.

[pozhidaevak]

Thanks, @nik9000.
It seems that _update_by_query uses static function RestUpdateByQueryAction::parseScript(Map<String, Object>) to parse script tag. This function is used only in this place.
I haven't found parsing of script field in RestUpdateAction class.
I'll try to find the solution that reuses the same code as in other parts of the project.
For now, I think the best way is to initialize XContentParser and use Script.parse() function.

[dakrone]

This should be closed by #26841