export.rake

#put in <project_directory>/lib/tasks
namespace :export do
  desc "Export ActiveRecord model records to a CSV file. Pass model name: model=ModelName"
  task :to_csv => :environment do

    require 'exportable'    
    model_name = ENV['model']
    open( "#{::Rails.root}" + '/log/export.pid', "w") do |file|
      file << Process.pid
    end
    File.open( "#{::Rails.root}" + "/tmp/#{model_name}.csv", "w" ) { |f|
      f << model_name.constantize.send( :to_csv )
    }    

    File.unlink( "#{::Rails.root}" + '/log/export.pid')
  end
end

exportable.rb

#
# inspired by
# http://www.brynary.com/2007/4/28/export-activerecords-to-csv
# put in <project_directory>/lib
require 'csv'

class ActiveRecord::Base
  def self.to_csv(*args)
    find(:all).to_csv(*args)
  end
  
  def export_columns(format = nil)
    ['id']  + self.class.content_columns.map(&:name)
  end
  
  def to_row(format = nil)
    export_columns(format).map { |c| self.send(c) }
  end
end

class Array
  def to_csv(options = {})
    if all? { |e| e.respond_to?(:to_row) }
      header_row = first.export_columns(options[:format]).to_csv
      content_rows = map { |e| e.to_row(options[:format]) }.map(&:to_csv)
      ([header_row] + content_rows).join
    else
      CSV.generate_line(self, options)
    end
  end
end