online_exam.rb

class OnlineExamController < ApplicationController
  def index
    @user=current_user
    @employee=Employee.find_by_employee_number(@user.username)
#   flash[:notice]="#{@employee.id}"
    @subjects=EmployeesSubject.find_all_by_employee_id(@employee.id)
    @subject_name=Array.new
    @batch_details=Array.new
    @display_details=Array.new
    @count=0
    for @t in @subjects
      @subject_name[@count]=Subject.find(@t.subject_id,:select=>'name,code,batch_id')
      @batch_details[@count]=Batch.find(@subject_name[@count].batch_id,:select =>'name')
      @display_details[@count][0]=@subject_name[@count].name+" "+@batch_details[@count].name
      @display_details[@count][1]=@t.subject_id
      @count+=1
    end
  end
end

#view
</div>

<div id="page-yield">
  <% unless flash[:notice].nil? %>
    <p class="flash-msg"> <%= flash[:notice] %> </p>
  <% end %>
<div class="box">

  <div class="label-field-pair">
   <label for="student_course">Select a batch:
<%= image_tag("loader.gif",:align => "absmiddle",:border => 0,:id => "loader", :style =>"display: none;" ) %>
   </label>
   <div class="text-input-bg">
<%= select :subject,:id,
        @display_details.map {|b| [b[0],b[1]]},
    {:prompt => 'selecct subject'},
    {:onchange =>"#{remote_function(
     :url=>{:action => 'show'},
     :before=> "Element.show('loader')",
     :success => "Element.hide('loader')" )}"}%>

</div></div>

<div id="subjects"></div>
<div id="modal-box" style="display:none;"></div>

<div class="extender"></div>
 </div></div>
~              
#error
You have a nil object when you didn't expect it!
You might have expected an instance of ActiveRecord::Base.
The error occurred while evaluating nil.[]=
/var/www/admin/app/controllers/online_exam_controller.rb:14:in `index'
/var/www/admin/app/controllers/online_exam_controller.rb:11:in `each'
/var/www/admin/app/controllers/online_exam_controller.rb:11:in `index'

When i comment that line it works properly and i tried in console it returns the value.