Upon upgrading to v2.11.2 of the `mongo` gem, our software's uses of `upsert: true` failed to insert new documents. Example:

collection.find.count # => 0
collection.find(_id: BSON::ObjectId('5dfa8ad7ea64ae457941102b')).update_one({'$set' => {foo: 'bar'}}, upsert: true) # => 
#<Mongo::Operation::Update::Legacy::Result:0x70339487480360 documents=[]>
collection.find.count # => 0

 
 
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).