How to update a document with new attribute

Question

I'm using ArangoDB in order to update a document with a new attribute.

My steps are as follows:

1. Get the document and store it in a variable

   doc = self.db.get_document(document_name="document_name")

2. Iterate over the doc, and add to it the attribute, for e.g.:

        for idx, movie in enumerate(doc):
   
            print(movie)
   
            query = 'UPSERT { movie_id: @movie_id, scene_element: @scene_element}  INSERT\
                { movie_id: @movie_id, scene_element: @scene_element, index: @index, \
                    _key: @_key, _id: @_id, _rev: @_rev, url_path: @url_path, captions: @captions, ref: @ref, \
                    experts: @experts, groundtruth: @groundtruth, base: @base, source: @source, File: @File } UPDATE \
                {   index: @index \
                } IN document_name'
   
            movie["index"] = idx
            self.pdb.aql.execute(query, bind_vars=movie)
   

And this works, I'm trying to use this query differently without the INSERT command. Why something like this doesn't work:

query = 'UPSERT { movie_id: @movie_id, scene_element: @scene_element}  UPDATE \
                    {   index: @index \
                    } IN document_name'

Finding the correct movie_id and scene_element, and just updating it with new index.

The error I'm getting is invalid syntax, and it asks for me to put the INSERT command.