Streaming a file into mongodb with node.js

I am trying to create a node.js program that accepts a file via http POST and writes it to mongodb. To keep memory usage down I'm using req.on('data') to receive the data in chunks.

The problem is that no matter how I approach the problem I can't get round the fact that the data events get fired before mongo has written the document.

This is what I've tried to do:

1. Post connection received, create document in mongo storing the _id from mongo.
2. Then each data event would update the document (using the _id)
3. When the end event is received I'd update the document setting some complete flag.

However, before mongo returns in step 1, data events start being fired.

Is there a correct way to do this? I'd rather not have to resort to writing to file.

Thanks, Kev.