Run rake task in rspec test

rake_spec.rb

require 'rake'

# make sure to set correct relative path 
load File.expand_path("../../../../lib/tasks/once.rake", __FILE__)

describe 'some name' do
  before(:all) do
    # because rake task accepts different environment
    Rake::Task.define_task(:environment)
    
    #replace 'once:create_tags' with your own rake task
    Rake::Task['once:create_tags'].invoke
  end
end