ログイン処理をできるはずが、internal server error になってしまう。

エラーログ

> Started POST "/session" for ::1 at 2015-05-20 19:25:08 +0900
> Processing by Staff::SessionsController#create as HTML   Parameters:
> {"utf8"=>"✓",
> "authenticity_token"=>"aewfM4fLXMzzudio329EdFoUcx1Y6vgwflK33aF+QijyR/srw1C2aABiPAHfpF8j8gJY3ToCUcz56AijGaGB+Q==",
> "staff_login_form"=>{"email"=>"taro@example.com",
> "password"=>"[FILTERED]"}, "commit"=>"ログイン"}   StaffMember Load
> (0.1ms)  SELECT  "staff_members".* FROM "staff_members" WHERE
> "staff_members"."email_for_index" = ? LIMIT 1  [["email_for_index",
> "taro@example.com"]]
> 
>   Rendered errors/internal_server_error.html.erb within layouts/staff
> (0.1ms)   Rendered staff/shared/_header.html.erb (0.2ms)   Rendered
> shared/_footer.html.erb (0.0ms) Completed 500 Internal Server Errorin
> 33ms (Views: 31.8ms | ActiveRecord: 0.1ms）

sessions/new.html.erb

<% @title = 'ログイン' %>

<div id="login-form">
  <h1><%= @title %></h1>

  <%= form_for @form, url: :staff_session do |f| %>
    <div>
      <%= f.label :email, 'メールアドレス' %>
      <%= f.text_field :email %>
    </div>
    <div>
      <%= f.label :password, 'パスワード' %>
      <%= f.password_field :password %>
    </div>
    <div>
      <%= f.submit 'ログイン' %>
    </div>
  <% end %>
</div>

sessions_controller.erb

class Staff::SessionsController < Staff::Base
    def new
        if current_staff_member
            redirect_to :staff_root
        else
            @form = Staff::LoginForm.new
            render action: 'new'
        end
    end

    def create
        @form = Staff::LoginForm.new(params[:staff_login_form])
        if @form.email.present?
            staff_member = StaffMember.find_by(email_for_index: @form.email.downcase)
        end

        if Staff::Authenticator.new(staff_member).authenticator(@form.password)
            session[:staff_member_id] = staff_member.id
            flash.notice = 'ログインしました。'
            redirect_to :staff_root
        else
            render action: 'new'
            flash.now.alert = 'メールアドレスまたはパスワードが正しく在りません。'
        end
    end

    def destroy
        session.delete(:staff_member_id)
        flash.notice = 'ログアウトしました'
        redirect_to :staff_root
    end
end

controllers/staff/base.rb

class Staff::Base < ApplicationController
    private
    def current_staff_member
        if session[:staff_member_id]
            @current_staff_member ||=
                StaffMember.find_by(id: session[:staff_member_id])
        end
    end
    helper_method :current_staff_member
end

forms/login_form.rb

class Staff::LoginForm
    include ActiveModel::Model
    attr_accessor :email, :password
end

services/staff/authenticator.rb

class Staff::Authenticator
    def initialize(staff_member)
        @staff_member = staff_member
    end

    def authenticate(raw_password)
        @staff_member &&
          !@staff_member.suspended? &&
          @staff_member.hashed_password &&
          @staff_member.start_date <= Date.today &&
          (@staff_member.end_date.nil? || @staff_member.end_date > Date.today) &&
          BCrypt::Password.new(@staff_member.hashed_password) == raw_password
    end
end

models/staff_member.rb

class StaffMember < ActiveRecord::Base
    before_validation do
        self.email_for_index = email.downcase if email
    end

    def password=(raw_password)
        if raw_password.kind_of?(String)
            self.hashed_password = BCrypt::Password.create(raw_password)
        elsif raw_password.nil?
            self.hashed_password = nil
        end
    end
end

config/routes.rb

Rails.application.routes.draw do
  namespace :staff, path: '' do
    root 'top#index'
    get 'login' => 'sessions#new', as: :login
    post 'session' => 'sessions#create', as: :session
    delete 'session' => 'sessions#destroy'
  end
end