ar_ext.rb

# in config/initializers

ActiveRecord::Relation.class_eval do
  include EachRow
end

each_row.rb

module EachRow
  def each_row &block
    return to_enum(:each_row) unless block_given?

    sql = to_sql
    res = connection.execute(sql)
    res.each do |row|
      yield row
    end

    res.clear
    nil
  end

  # WARNING: This is way too clever for anyone's good.
  # Paramaters passed depend on the name you give them in the block.
  #
  # Example:
  # lazy_map_rows do |id, updated_at|
  #   "#{self.class} with id #{id} was last updated at #{updated_at}"
  # end
  def lazy_map_rows mapper = nil, &block
    return to_enum(:lazy_map_rows, block) unless mapper
    return to_enum(:lazy_map_rows, mapper) unless block

    table = arel_table
    query = except(:select)

    cols = mapper.parameters.map do |(_, col)|
      query = query.select table[col]
      col.to_s
    end

    proc = lambda do |row|
      mapper.call *row.values_at(*cols)
    end

    sql = query.to_sql
    res = connection.execute(sql)
    res.each { |row| yield proc.call(row) }
    res.clear

    nil
  end
end

examples.rb

# NOTE: #each_row and #lazy_map_rows probably only work with Postgres.
# NOTE: #lazy_map_rows most likely only works on MRI 1.9.3 or later.

# Assume a model defined as so:
# Tag(id: integer, name: string, created_at: datetime, updated_at: datetime)
class Tag < ActiveRecord::Base
  # EachRow methods can be called on any scope.
  # Use .scoped or .unscoped if you don't use query methods first.
end

# And the associated table has the following data:
# [#<Tag id: 1, name: "socks",     created_at: "2012-02-27 22:40:46", updated_at: "2012-02-27 22:40:46">,
#  #<Tag id: 2, name: "hateful",   created_at: "2012-02-27 22:40:46", updated_at: "2012-02-27 22:40:46">,
#  #<Tag id: 3, name: "streamage", created_at: "2012-03-16 15:39:05", updated_at: "2012-03-16 15:39:05">,
#  #<Tag id: 4, name: "empty",     created_at: "2012-03-20 13:58:46", updated_at: "2012-03-20 13:58:46">]

# Each of the following examples will perform ONLY 1 iteration of the rows in the table.

tags = Tag.scoped.with_whatever.you_want
tags.lazy_map_rows { |id, name| "#{id} => #{name}" }.to_a
# => ["1 => socks", "2 => hateful", "3 => streamage", "4 => empty"]

tags.lazy_map_rows { |created_at, name| "#{name} was created at #{created_at}" }.to_a
# => ["socks was created at 2012-02-27 22:40:46.426997",
#     "hateful was created at 2012-02-27 22:40:46.447398",
#     "streamage was created at 2012-03-16 15:39:05.242834",
#     "empty was created at 2012-03-20 13:58:46.88798"]

# Please Enjoy Responsibly!