我建了一个登录页面【index.html.erb】：

<%= form_tag do %>
    <fieldset>
    <legend>Please Input Details</legend>
      <div>
        <label for="username">username:</label>
        <%= text_field_tag :username, params[:username] %>
      </div>

      <div>
        <label for="password">password:</label>
        <%= password_field_tag :password, params[:password] %>
      </div>

      <div>
        <%= submit_tag "Login" %>
      </div>
    </fieldset>
<% end %>

用以下后台代码进行登录验证【welcome_controller.erb】:

class WelcomeController < ApplicationController
  def index
    if "Vincent" == params[:username] && "123456" == params[:password]
        redirect_to(welcome_login_path)
    else
        redirect_to(welcome_logout_path)
    end
  end

  def login
  end

  def logout
  end
end

结果只会跳至http://localhost:3000/welcome/logout，请问我的代码哪里出错了？