Rails provides a set of view helpers for including assets on a page: image_tag, javascript_include_tag, and stylesheet_link_tag. These make use of your host domain as set in your Rails configuration via config.action_controller.host.

Those helpers are pretty clever - you can specify a full URL, or you can just give it a string like front and the helper will expand that to http://my.website/assets/javascripts/front.js. Better yet, if you’re using the Rails asset pipeline, the latter case will make sure to append the generated MD5 hash to grab the correct version of the file.

Where the helper fails is if you are using the asset pipeline, and want to point a particular helper at a resource hosted on a different domain to the one you configured, but still using the asset pipeline hash. (Don’t ask me why I needed to do this at Travelstart, but I did.)

If you find yourself challenged to do this, the asset_url helper comes to your rescue:

javascript_include_tag(asset_url('front', host: 'special_resource.website'))