Collection#insert should return the "_id"

[mwlang]

Currently Collection#insert returns nil.

It would be helpful for building compound documents if, when inserting a new document, Collection#insert returned either mongo document with the "_id" or just the "_id" field value so that the collection doesn't have to be re-queried to retrieve the value.

[durran]

It would be nice, but unfortunately Mongo doesn't give us anything back when inserting (since it's fire and forget), and when in safe mode it still doesn't give the id back if it generated it on the server. So there really isn't any possible way for us to do this unless it was a core feature in MongoDB.