active_record_each_by.rb

module ActiveRecordEachBy
  # This method lets you iterate over the results of a .find, in groups.
  # (Basically an interface to LIMIT.)
  # Anything you can pass as options to .find, you can pass here. 
  # Example 1:
  #   Order.each_by(100, :conditions => { :cc_processed_at => nil }) do |order|
  #     # do stuff with order
  #   end
  # Example 2:
  #   Person.each_by(50, :order => 'name') do |person, index|
  #     # do stuff with person and index
  #   end
  # Pass :update => true in the options to print a message before each group is
  # fetched from the db.
  #
  # Author: Elliot Winkler <elliot.winkler@gmail.com>
  # Source: http://snippets.dzone.com/posts/show/5461
  def each_by(group_size, options={}, &blk)
    update = options.delete(:update) || false
    num_records = count(options.except(:from))
    return 0 if num_records == 0
    #raise "Number of records: #{num_records}"
    also_pass_offset = (blk.arity == 2)
    0.step(num_records, group_size) do |offset|
      find_options = { :offset => offset, :limit => group_size }.merge(options)
      if update
        if num_records == 1
          puts ">> Reading the only record."
        else
          start_offset = offset + 1
          end_offset   = offset + group_size
          end_offset   = num_records if num_records < end_offset
          puts ">> Reading records #{start_offset}-#{end_offset}."
        end
      end 
      find(:all, find_options).each do |record|
        also_pass_offset ? blk.call(record, offset) : blk.call(record)
      end
    end
    num_records
  end
end

ActiveRecord::Base.extend(ActiveRecordEachBy)