ruby-on-rails - レールの単純なフォームはデータを投稿できません

Question

レールではシンプルなフォームを使用しています。データ入稿後。ページは常に application_form#new に戻り、データは db に保存できません。なんで？

ログ

Started POST "/application_forms" for 127.0.0.1 at 2013-05-25 19:23:28 +0800
Processing by ApplicationFormsController#create as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"yypWFcsXbdSVtp3WXUZKvstzHWsV0OOpLoGSxO1ldJ0=", "application_form"=>{"student_name"=>"somebody", ...... ,"application_type"=>"b"}, "commit"=>"Submit"}
  [1m[35mUser Load (0.0ms)[0m  SELECT "users".* FROM "users" WHERE "users"."id" = 1 LIMIT 1
  [1m[36m (0.0ms)[0m  [1mbegin transaction[0m
  [1m[35m (0.0ms)[0m  rollback transaction
  Rendered application_forms/_form.html.erb (24.0ms)
  Rendered application_forms/new.html.erb within layouts/application (25.0ms)
Completed 200 OK in 61ms (Views: 52.0ms | ActiveRecord: 0.0ms)

意見

<%= simple_form_for @application_form, :html => {:class => 'form-horizontal' } do |f| %>

  <legend>Basic Info</legend>
  <%= f.input :student_name %>
  <%= f.button :submit, 'Submit', :class => 'btn-large btn-primary'%>

<% end %>

---

コントローラ

 def create

    @application_form = ApplicationForm.new(params[:application_form])

    respond_to do |format|
      if @application_form.save
        format.html { redirect_to @application_form, :notice => 'Submit ok.' }
      else
        format.html { render :action => "new" }
      end
    end
  end