fixed 500 on photos#show for private photos and logged out user

app/controllers/photos_controller.rb

@@ -144,7 +144,7 @@ class PhotosController < ApplicationController
     if user_signed_in?
       @photo = current_user.find_visible_shareable_by_id(Photo, params[:id])
     else
-      @photo = Photo.where(id => params[:id], :public => true)
+      @photo = Photo.where(:id => params[:id], :public => true).first
     end
 
     if @photo