Trying to load a non-existent document will keep it’s ID in the object. 

You can reuse this object to create a new one...

This will create/update the document {_id: 123, foo: 'bar'}. (previously, the document would have an auto generated id).
```
$doc = new SomeDoce(123);
$doc->load();
$doc->foo = 'bar';
$doc->save();
```