An identical question has been asked before AssetTagHelper::image_path outside views but the solution does not work for Rails 3.
I have a requirement where I am returning some data using JSON format and part of the data needs to return the full path of the image. I am building the response in the controller and calling render :json. I am also specifying an asset_host in the environment.rb so I need a way to include that in the returned data from inside a Rails controller.