async: NameError: uninitialized constant inside Async::Task

I don’t seem to have scope for my ActiveRecord model inside an Async::Task.

For my model named F I get:

NameError: uninitialized constant F

My implementation:

Async do
  # Queue of up to 10 items:
  items = Async::LimitedQueue.new(10)

  # Five producers:
  5.times do
    Async do |task|
      # while true
      while F.unchecked.count.positive?
        # t = rand
        f = F.unchecked.first
        f.do_checking!
        t = check_f(f)

        task.sleep(t)
        items.enqueue(t)
      end
    end
  end

  # A single consumer:
  Async do |task|
    while item = items.dequeue
      puts "dequeue -> #{item}"
    end
  end
end

I also tried using ::F but got the same error.