Currently there is an assumption in the driver that a $out stage looks like this:

{ $out : "collectionName"}

and therefore that the driver can open a cursor on that collection.

But we will be adding support for something like this:

{ $out : {s3: { ...  } }

We can't necessarily even open a cursor on the S3 bucket.

I suspect the best way to handle this is to introduce a new Aggregate method that doesn't return a cursor. Perhaps call it AggregateTo. And than method should remove any assumptions about the contents of a $out (or $merge) stage.