How to save subdocument withought overwriting the existing sub document data?

[ptheofan]

Hi, excuse my question if it sounds trivial, but I'm trying to figure out how to save a subdocument withought overwriting the existing subdocument data whilst the data were not fetched.

I have tried with doc->sub1->sub2 = new Document() but it unfortunately overrides the existing data. Is there any way I can do that operation whithought having to fetch the subdocument in the first place?

Pretty thanks :)