Helper for testing rake tasks in rails using minitest.

awesome_report_test.rb

require 'test_helper'

# testing rake task app:awesome_report
# defined in file lib/tasks/app/awesome_report.rake

describe 'App::AwesomeReportTaskTest' do
  it 'generates the awesomeness report' do
    subject.invoke
    assert File.exists?('awesomeness_report.csv')
  end
end

rake_task_test_case.rb

# require this in test_helper

class RakeTaskTestCase < ActiveSupport::TestCase
  before do
    require 'rake'
    Rake::Task.define_task :environment
    load rake_task_load_path
  end

  private
  def subject
    Rake::Task[task_name]
  end

  def rake_task_load_path
    "#{_tasks_base_path}/#{_relative_task_file_path}"
  end

  def task_name
    _relative_task_file_path.sub('/', ':').sub(/\.rake$/, '')
  end

  def _relative_task_file_path
    "#{self.class.name.sub(/TaskTest$/, '').underscore}.rake"
  end

  def _tasks_base_path
    File.expand_path("#{Rails.root}/lib/tasks")
  end
end

class MiniTest::Spec
  register_spec_type(/.*TaskTest/, RakeTaskTestCase)
end

Rails 5

require 'rake'
APPNAME::Application.load_tasks
Rake::Task["task_name"].invoke