下拉列表表单验证警报在Rails中的ruby中

Question

I have drop down list and two radio button in a form.here is the code.i dont know how to set validations in controller.when not selecting value from drop down and one of the radio button,i have to show the warning messages 'please select value from dropdown' and check one radio button filed.how it is possible in ruby on rails while submitting a form.

<%= form_tag :action => 'show' do %>
    <strong>Select device: </strong> <%= collection_select(:device, :id, @devices, :id, :name, options ={:prompt => "-Select a device"}) %>
    <br></br>
    <strong>Chose: </strong><%=  radio_button_tag :name,:time, false, :onclick => "this.parentNode.submit();"%>Time
    <%=  radio_button_tag :name,:graph%>Graph
    <% end %> 

In device.rb i set the following but not showing any messages.

 class Device < ActiveRecord::Base
   attr_accessible :name
   validates_presence_of :name
   has_many :properties

   def validate
     if name == 'None'
       errors.add_to_base("You must select a device name")
     end
   end
 end

Answer 1

Why not use a validates_exclusion_of ? (or its equivalent in Rails 3)

http://apidock.com/rails/v2.3.8/ActiveModel/Validations/ClassMethods/validates_exclusion_of