NoMethodError in ClassifiedController#update

My project was working when I add new field category
It is showing the error in create form and edit form
i am sending u controller file as attechment
this is my model(category.rb) dor category file
class Category < ActiveRecord::Base

has_many :classifieds

end
and this is edit.rhtml

Editing Classified: <%= @classified.title -%>

<%= form_tag :action => 'update', :id => @classified %>

Title
<%= text_field 'classified', 'title' %>

Price
<%= text_field 'classified', 'price' %>

Location
<%= text_field 'classified', 'location' %>

Description
<%= text_area 'classified', 'description' %>

Category
<%= collection_select(:classified, :category_id, @categories, :id, :name) %>

Email
<%= text_field 'classified', 'email' %>

<%= submit_tag "Save changes" %> <%= form_tag %> <%= link_to 'Back', {:action => 'list' } %>