gistfile1.rb

class PostsController < ApplicationController
  ...
  # Displays posts that contain any of one or more tags passed in
  # GET /posts/tag/:tag1,:tag2,... || GET /posts/tag/:tag
  def tag
    @tag = params[:tag]
    @tag = @tag.split(",") if @tag =~ /,/
    @posts = Post.all(:conditions => { :tags => @tag }, :order => 'created_at desc' )

    respond_to do |format|
      format.html
      format.xml { render :xml => @posts }
    end
  end
end