mongodb cursors on indexes and jumping

mongodb_cursor_jump.py

#context: i want to mergejoin two generators/cursors by a key of a mongo document


#assuming an compund index af  "keyA":1,"keyB":1,"keyC":1 exists.

#this is how i do it
cursor = self.collection.find( { "keyA":"foo", "keyB":"bar" }, { "keyC":1})
nextid = None
while 1:
    mongo_doc = cursor.next()
    result = mongo_doc["keyC"] 
    if nextid and nextid > result:
       mongo_doc.update( {"keyC":{"$gte":nextid}}) 
       cursor = self.collection.find(mongo_doc, {"keyC":1})
    #this is where my merge join operation sends back the next possible lowest key
    nextid = yield( result)


#this is how it should look imho
cursor = self.collection.find( { "keyA":"foo", "keyB":"bar" }, { "keyC":1})
for mongo_doc in cursor:
    result = mongo_doc["keyC"]
    nextid = yield( result)
    if nextid:
        cursor.jump_to( {"keyC":{"$gte":nextid}} )