How to test rake tasks with RSpec

spec-lib-tasks-notification_rake_spec.rb

# spec/lib/tasks/notification_rake_spec.rb
require 'spec_helper'

describe 'notifications:weekly' do
  include_context "rake"

  let(:users){ [double(:user)] }
  let(:user_deliveries){ double(:user_deliveries) }

  before{ User.stub(:all).and_return( users ) }

  it do
    UserDeliveries.should_receive(:new).once.with(users).and_return(user_deliveries)
    user_deliveries.should_receive(:add_event_deliveries)
    # ... 

    subject.invoke
  end
end

spec-support-shared_contexts-rake.rb

# spec/support/shared_contexts/rake.rb
require "rake"

shared_context "rake" do
  let(:rake)      { Rake::Application.new }
  let(:task_name) { self.class.top_level_description }
  let(:task_path) { "lib/tasks/#{task_name.split(":").first}" }
  subject         { rake[task_name] }

  def loaded_files_excluding_current_rake_file
    $".reject {|file| file == Rails.root.join("#{task_path}.rake").to_s }
  end

  before do
    Rake.application = rake
    Rake.application.rake_require(task_path, [Rails.root.to_s], loaded_files_excluding_current_rake_file)

    Rake::Task.define_task(:environment)
  end
end

stolen from http://robots.thoughtbot.com/post/11957424161/test-rake-tasks-like-a-boss