How to configure an extra/different migrations folder

Update for Rails 5/6;

Rails 5 recommends setting additional migration paths in your config/database.yml file. It's very easy, see this example;

development:
  migrations_paths:
  - "db/migrate/other_db"
  - "db/migrate/something_else"

ActiveRecord::Migrator.migrations_path= will be deprecated in Rails 6.

In your config file (config/application.rb for all environments or config/environments/$(environment).rb only for particular environment) add this line:

config.paths['db/migrate'] += 'db/migrate/other_db'

And if you want to change default 'db/migrate' path (config.paths['db/migrate'] is an array with one string 'db/migrate' by default), do this:

config.paths['db/migrate'] = ['db/my_migrate']

Here are default config.paths, which we also can change:

"app" => ["app"],
"app/assets" => ["app/assets"],
"app/controllers" => ["app/controllers"],
"app/helpers" => ["app/helpers"],
"app/models" => ["app/models"],
"app/mailers" => ["app/mailers"],
"app/views" => ["app/views"],
"lib" => ["lib"],
"lib/assets" => ["lib/assets"],
"lib/tasks" => ["lib/tasks"],
"config" => ["config"],
"config/environments" => ["config/environments"],
"config/initializers" => ["config/initializers"],
"config/locales" => ["config/locales"],
"config/routes" => ["config/routes.rb"],
"db" => ["db"],
"db/migrate" => ["db/migrate"],
"db/seeds" => ["db/seeds.rb"],
"vendor" => ["vendor"],
"vendor/assets" => ["vendor/assets"],
"vendor/plugins" => ["vendor/plugins"],
"config/database" => ["config/database.yml"],
"config/environment" => ["config/environment.rb"],
"lib/templates" => ["lib/templates"],
"log" => ["log/development.log"],
"public" => ["public"],
"public/javascripts" => ["public/javascripts"],
"public/stylesheets" => ["public/stylesheets"],
"tmp" => ["tmp"],

By the way, if you are building a gem to work with Rails, you can place a block like the following in your rail tie to add the gem's own migrations.

  root = ... # the path to your gem
  initializer :append_migrations do |app|
    unless app.root.to_s.match root
      app.config.paths["db/migrate"] << File.join(root, 'db/migrate')
    end
  end

There is no need to copy migrations from your gem with generators if you use this technique.

You can make a method to yield the root directory of your gem with some thing like this...

module MyGemName
  def root
    File.expand_path '../..', __FILE__
  end

  module_method :root
end

... in the file lib/my_gem_name.rb in your gem.

based on the answer by Swanand, we can write a migration to load the migrations in an external dir:

class MigrateMetadata < ActiveRecord::Migration
  MIGRATIONS_PATH='db/migrate/metadata'
  def self.up
   Dir["#{MIGRATIONS_PATH}/[0-9]*_*.rb"].
   sort.map{|filename|require filename}.flatten.
   each{|class_name| const_get(class_name).up}
  end

  def self.down
    Dir["#{MIGRATIONS_PATH}/[0-9]*_*.rb"].sort.reverse.
    map{|filename|require filename}.flatten.
    each{|class_name| const_get(class_name).down}
  end
end