search

auctions.html.erb

<%= form_tag('/auctions',:method=>:get) do %>                          
<%= select_tag "category_id", options_from_collection_for_select(Category.all, "id", "name") %>    
 <%= submit_tag %>
<% end %>
#In your controller
def index
 if params[:category_id]
  category=  Category.find(params[:category_id])
  @auctions = category.auctions
 else
 @auctions = Auctions.all
 end
end