image - How to get the actual file path (to use with send_file) when using Rails' asset pipeline -

i'm using controller in rails 4 app serve images, want restrict access (similar this answer). images being served not stored in asset pipeline themselves, , programmatically generated. if image hasn't been generated yet, want show default / fallback image user. image stored other static assets.

so how can actual path image file give send_file in controller? noted here asset_path , image_path helpers not give local filesystem path, because of asset pipeline fingerprinting stuff, can't construct path myself either.

class thingscontroller    before_filter :authorize_for_thing    def show     respond_to |format|       format.all {         if @thing.graph.file.present?           file = @thing.graph_url()         else       ### serve static default image if file isn't there: ###           file = file.join(rails.root,            actioncontroller::base.helpers.asset_path("default_graph.png")           )         end         send_file file, :type=>"image/png", :disposition => 'inline'       }     end   end end 

but asset_path("default_graph.png") give of form:

assets/default_graph[possible-md5].png 

when file lives elsewhere, , send_file needs literal path file.

rails.application.assets_manifest.assets[name]