The Mocha library is the one Rails uses for its own testing, so it seems the natural choice for a test-double library to use with Minitest. Full Mocha docs are available at http://gofreerange.com/mocha/docs/.
Mocha needs to be installed after Minitest, which requires a slight indirection to ensure your Rails app does the right thing.
First, in the Gemfile’s :test group, add the Mocha gem:
gem "mocha", require: false
The require: false ensures that Mocha loads in the correct order.
Then, inside the test/test_helper.rb file, you manually require Mocha at any point in the file after the rails/test_help library is loaded:
At that point we should be good to go. Please see Chapter 7, Using Test ...