rails_asset_id

# File actionpack/lib/action_view/helpers/asset_tag_helpers/asset_paths.rb, line 71
        def rails_asset_id(source)
          if asset_id = ENV["RAILS_ASSET_ID"]
            asset_id
          else
            if self.cache_asset_ids && (asset_id = self.asset_ids_cache[source])
              asset_id
            else
              path = File.join(config.assets_dir, source)
              asset_id = File.exist?(path) ? File.mtime(path).to_i.to_s : ''

              if self.cache_asset_ids
                add_to_asset_ids_cache(source, asset_id)
              end

              asset_id
            end
          end
        end