How To Test Rake Task

How to test a rake task

Annotated code below. Even though the example is tied to Rails it should be pretty straightforward to copy to other testing tools (RSpec, plan Minitest, you name it).

require "test_helper"
require "rake"

class MyRakeTaskTest < ActiveSupport::TestCase
  setup do
    # By requiring rake we can use its helper to load the file with the tasks
    # we want to test. Another way to load it would be via `load` as follows
    # 
    # load File.expand_path("../lib/tasks/my_rake_task.rake", __FILE__)
    # 
    # For Rails users
    # Rails.application.load_rake_tasks
    # would also work, but that is a heavy-weight operation and I'd avoid that.
    Rake.application.rake_require "tasks/my_rake_task"

    # If your task depends on other tasks you should load them as well. In Rails it is typical
    # to depend on `environment` (which happens to be loaded in test already, we are in some env
    # after all), so defining a "mock" is more than enough.
    Rake::Task.define_task(:environment)
    
    # reenable - without that, you can't run the task more than once. I mean you can, but it won't
    # be reevaluated, and you'll be served with cached content.
    Rake::Task["my_rake_task"].reenable
  end

  test "very important side effect" do
    Rake::Task["my_rake_task"].invoke
    # Happy testing!
  end
end