Can't stub with mocha in ruby on rails test - ruby-on-rails

Question

I'm fairly new to Ruby/Ruby on Rails and having trouble stubbing out a method via mocha in an existing codebase.
I've simplified the code down to a MWE where this breaks.
Here is test_helper.rb:
ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
require "rails/test_help"
class ActiveSupport::TestCase
end
class Minitest::Test
def before_setup
end
end
And here is the test:
require 'test_helper'
require 'mocha/minitest'
class MyTest < ActionMailer::TestCase
describe "some test" do
it "should stub" do
My::Class.stubs(:bar).returns("foo")
puts My::Class.bar
end
end
end
This results in the following error when I run the test:
Mocha::NotInitializedError: Mocha methods cannot be used outside the context of a test
However, when I redefine my test_helper.rb as follows:
ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
require "rails/test_help"
class ActiveSupport::TestCase
end
# class Minitest::Test
# def before_setup
#
# end
# end
The test passes (and "foo" is printed as expected).
Why does the class Minitest::Test...end in test_helper.rb cause the first error? I can't remove that code from the actual codebase, so how can I modify it to work with mocha?
Ruby version: 2.4.1
Rails version: 4.2.8
Mocha version: 1.5.0

Answer 1

Adding a call to super in the patched method before_setup in test_helper.rb works:
ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
require "rails/test_help"
class ActiveSupport::TestCase
end
class Minitest::Test
def before_setup
# do something
super
end
end
This call to super allows the before_setup of Mocha::Integration::MiniTest to be called, which is necessary for proper initialization.