Methods for migrating devise passwords from naive MD5 to a new strategy

migrate-devise-passwords.rb

class User < ActiveRecord::Base

  alias_method :valid_password_without_legacy?, :valid_password?
  def valid_password?(incoming_password)
    if valid_password_without_legacy?(incoming_password)
      return true
    elsif Digest::MD5.hexdigest(incoming_password) == self.encrypted_password
      update_legacy_password(incoming_password)
    end
  end

  def update_legacy_password(incoming_password)
    self.password = incoming_password
    self.encrypted_password = password_digest(@password)
    save!
  end

end