Your original error comes from our verifying doubles feature, which you must have turned on as part of the upgrade process, you can turn it off if Mongoid isn't playing nicely with it with:
RSpec.configure do |config|
config.mock_with :rspec do |mocks|
mocks.verify_partial_doubles = false
Both Phils, and yours, snippets pass in an empty Rails app, so its likely an interaction between Rails and other libraries you are using.