mongoid_criteria.rb

module Mongoid
  class Criteria

    def each_by(by, &block)
      idx = 0
      total = 0
      set_limit = options[:limit]
      while (results = ordered_clone.limit(by).skip(idx)) && results.any?
        results.each do |result|
          return self if set_limit and set_limit >= total
          yield result
          total += 1
          idx += 1 unless result.nil? or result.deleted?
        end
      end
      self
    end

    private

    def ordered_clone
      options[:sort] ? clone : clone.asc(:_id)
    end

  end
end

Added support for when a record is deleted the index counter is not incremented, preventing skipping when iterating over a collection that is being trimmed of records. Note that still if you are adding records you might get the opposite - records iterated twice.