Updating nested documents in mongodb

Question

Say I have a data structure something like this:

{
    'name': 'test',
    'anotherdoc': {
        'something': 'someval',
        'somenum': 1
    }
}

Now, say I wanted to set something. Initially, I though it would be done like so:

collection.update({'_id': myid}, {$set: {'anotherdoc.something': 'somenewval'});

This, however, seems to be incorrect. It does put some data in there, but it does so in an odd manner. It would, in this case, end up like so:

[
    {
        'name': 'test',
        'anotherdoc': {
            'something': 'someval',
            'somenum': 1
        }
    },
    ['anotherdoc.something', 'someval']
]

Of course, not what I was looking for.