Upon upgrading to v2.11.2 of the `mongo` gem, our software's uses of `upsert: true` failed to insert new documents. Example:
What we expect to happen is for a new document with keys _id and foo to be inserted.
What actually happens is no document is inserted.
Downgrading to v2.9.x fixes this problem. It appears related to a mismatch of symbol vs. string "upsert" keys in https://github.com/mongodb/mongo-ruby-driver/pull/1439 (https://jira.mongodb.org/browse/RUBY-1878).