--controller--
class SearchController < ApplicationController
  def search
     @query = params[:query]
     @posts = Post.search(@query)
     format.html { redirect_to(posts_url) }
  end
end


--view--

<% form_tag :controller => "search", :action => "index" do %>
  <p>
    <%= text_field_tag "query", "", "size" => 30 %>
  </p>
  <p>
    <%= submit_tag "Search" %>
  </p>
<% end %>