every_nth.rb

module EveryNth

  def every_nth(n)
    Enumerator.new do |yielder|
      with_index do |*a, i|
        yielder.yield(*a)

        yield i if (i+1) % n == 0
      end
    end
  end
end

Enumerator.send(:include, EveryNth)

example.txt

[1] pry(main)> 1000.times.every_nth(100) { |i| puts "Hello from #{i}" }.each { |n| nil }
Hello from 99
Hello from 199
Hello from 299
Hello from 399
Hello from 499
Hello from 599
Hello from 699
Hello from 799
Hello from 899
Hello from 999
=> 1000

Actual Use Case:

huge_mongo_collection.batch_size(100).every_nth(1000) { print_memory_usage }.each do |row|
  # Do actual work here
end