microposts_controller.rb

class MicropostsController < ApplicationController

  before_filter :set_micropost, :only => [:show, :edit, :update, :create, :new]
  before_filter :authenticate, :only => [:create, :destroy] # Same here?

  # Omitted.

  def destroy
    @micropost.destroy
    redirect_back_or root_path
  end

  def set_micropost
    # If this record can't be found, it will throw ActiveRecord::RecordNotFound,
    # which will cause rails to return a 404.

    # Now you don't need to check if the owner is the user, since we infer that here.
    @micropost = current_user.microposts.find(params[:id]) 
  end  
  private :set_micropost

end