add_index.rb

  desc 'generate  add_index declarations  for all foreign key fields'
  task :indexes => :environment do

    files = nil
    Dir.chdir(File.join(RAILS_ROOT, "app/models" )) do
      files = Dir["*.rb"]
    end

    models = []
    files.each do |m|
      class_name = m.sub(/\.rb$/, '').camelize
      begin
        klass = class_name.split('::').inject(Object){ |klass,part| klass.const_get(part) }
        if klass < ActiveRecord::Base && !klass.abstract_class?
          models << klass
        end
      rescue Exception
      end
    end

    res = models.map do |klass|

      table_name = klass.table_name

      indexed_columns = []
      res = klass.connection.execute("SHOW INDEX FROM #{table_name}")

      while row = res.fetch_row
        indexed_columns << row[4]
      end

      res = klass.columns.select{|col| col.name =~ /_id$/ }.reject{|col| indexed_columns.index col.name }.map do |column|
        "  add_index :#{table_name}, :#{column.name}"
      end

      res.blank? ? nil : res.join("\n")

    end.compact.join("\n")

    down = res.gsub(/add_index/, 'remove_index')
    puts %"def self.up\n#{res}\nend\n\ndef self.down\n#{down}\nend\n"

  end