Undefined method

I am very new to ROR. (Ruby 1.8.7, rails 2.1.1, rack
0.8.7, mysql 5.1.41)

I has my_app/lib/migration_helpers.rb file:
module MigrationHelpers
  def self.foreign_key(from_table, from_column, to_table)
    constraint_name = "fk_#{from_table}_#{to_table}"

    execute %{alter table #{from_table}
       add constraint #{constraint_name}
          foreign key (#{from_column})
          references #{to_table}(id)


In my_app/db/migrate/001_create_dummy.rb:

require "migration_helpers"
class CreateAirports < ActiveRecord::Migration
  extend MigrationHelpers

  def self.up
    create_table (:airports, :options => "ENGINE=InnoDB") do |t|
      t.integer :country_id, :null => false #foreign key
      t.string :code, :null => false, :limit => 3
      t.string :name, :null => false, :limit => 45

    foreign_key(:airports, :country_id, :countries)

  def self.down
    drop_table :airports

When I rake db:migration it got this error:
-- create_table(:airports, {:options=>"ENGINE=InnoDB"})
   -> 0.0156s
-- foreign_key(:airports, :country_id, :countries)
rake aborted!
undefined method `foreign_key' for

What did I miss?

Thank you


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