Generator

create_settings.rb

class CreateSettings < ActiveRecord::Migration
  def up
    create_table :settings do |t|
      t.string :paramkey, :limit => 255, :null => false
      t.string :paramvalue, :limit => 1000
      t.string :i18nkey, :limit => 35, :null => false
      t.string :paramtype, :limit => 25

      t.timestamps
    end

    change_table :settings do |t|
      t.index :paramkey, :unique => true
    end
  end

  def down
    change_table :settings do |t|
      t.remove_index :paramkey
    end

    drop_table :settings
  end
end

install_generator.rb

module Amazing
  class InstallGenerator < Rails::Generators::Base
    include Rails::Generators::Migration
    
    source_root File.expand_path("../templates", __FILE__)

    def install
      puts "Here we are in the plugin generator."
      migration_template "create_settings.rb", "db/migrate/create_settings.rb"
    end

  private
    def self.next_migration_number(number)
      ActiveRecord::Migration.new().next_migration_number 0
    end  
  end
end

Structure.txt

/lib
  /generators
    /Amazing
      /install
        /install_generator.rb
      /templates
        /create_settings.rb

Rails commands:

veytmbp:AppTester prevole $ rails g
Usage: rails generate GENERATOR [args] [options]

General options:
-h, [--help] # Print generator's options and usage
-p, [--pretend] # Run but do not make any changes
-f, [--force] # Overwrite files that already exist
-s, [--skip] # Skip files that already exist
-q, [--quiet] # Suppress status output

Please choose a generator below.

Rails:
assets
controller
generator
helper
integration_test
mailer
migration
model
observer
performance_test
plugin
resource
scaffold
scaffold_controller
session_migration

Amazing:
amazing:install

Coffee:
coffee:assets

Formtastic:
formtastic:install

Jquery:
jquery:install

Js:
js:assets

pveytmbp:AppTester prevole $ rails g amazing:install
Could not find generator amazing:install.