Devise install from existing model/database

I am wondering how can i add devise to an existing database with a different user. Here I already have a customer model define and I want to change to allow devise to work on it.

I have created a new migration and inserted the code has follow

class AddDeviseToCustomer < ActiveRecord::Migration
  def change
    change_table :customers do |t|
      #t.database_authenticatable
      t.string :encrypted_password, :null => false, :default => '', :limit => 128
      t.confirmable
      t.recoverable
      t.rememberable
      t.trackable
      t.token_authenticatable
      t.timestamps
    end
  end
end

According to this it should work. https://github.com/plataformatec/devise/wiki/How-To:-change-an-already-existing-table-to-add-devise-required-columns. But when running rake db:migrate i get the following

undefined method `confirmable' for #<ActiveRecord::ConnectionAdapters::Table:0x9286a28>

I have run the following line

rails g devise:install

Any reason devise won't recognize it, do i need to do something to say customer is a devise?? Thanks in advance