mongodb/mongo — pull request #371 by ashaffer (2013-02-04 21:35:53+00:00)
Title: Update src/mongo/db/ops/update_internal.cpp

If a document does not have an _id, then it should be legal to write to that field in an update. The need for this ability is documented in these two threads:

https://jira.mongodb.org/browse/SERVER-4175
https://jira.mongodb.org/browse/SERVER-340