Mongo conditional for "key doesn't exist"?

I want to find a document using a conditional if the key == None or if the key doesn't exist. Something like this:

myDoc = self.request.root.db.myDocs.find_one({
                          '$or': [
                              {'myKey' : $doesNotExist } ,
                              {'myKey' : None }
                            ]
                    })

I'd also like to be able to find a document just by a missing key like this:

myDoc = self.request.root.db.myDocs.find_one( {'myKey' : $doesNotExist } )

How can I accomplish this?