:pk is mentioned, but not supported

From the documentation linked from the Mongo web site, this should work:

class PKFactory
def create_pk(doc)
return doc if doc[:_id] or doc['_id']
doc[:_id] ||= SecureRandom::uuid()
doc
end
end

db = Mongo::MongoClient.from_uri(uri).db(database, pk: PKFactory.new)

However, it does not.