Mongoid extension used for iteration through MongoDB collection using ranges. Useful when iteration performing asynchronously. Works much faster then skip/limit queries.

criteria.rb

module Mongoid
  class Criteria
    def range_by(by = 1000, &block)
      raise ArgumentError, "Please specify correct range" unless by.to_i > 0
      
      first = klass.order_by(:_id.asc).first.id.generation_time
      pages = (klass.count.to_f/by.to_i).ceil
      frame = (
        (klass.order_by(:_id.asc).last.id.generation_time - first + 1).to_f /
        pages
      ).ceil
      
      pages.times { |x|
        yield clone, gt: (first + x*frame).to_i, lt: (first + (x+1)*frame).to_i
      }
    end
    
    def range_with(range)
      range.stringify_keys!
      
      clone.where(
        :_id.gte => BSON::ObjectId.from_time(Time.at(range["gt"])).to_s,
        :_id.lt => BSON::ObjectId.from_time(Time.at(range["lt"])).to_s
      )
    end
  end
end

example.rb

Post.all.range_by(500) { |criteria, range|
  # Immediatly find objects
  puts criteria.range_with(range).count
  
  # Or pass range hash to some async worker
  # Hash format: {:gt=>1355431169, :lt=>1355431170}
  PostWorker.perform_async(range)
}