Better Errors

Raise a descriptive error if specific config file `app/assets/config/manifest.js` is not found when using Sprockets 4.

The `link ../javascripts/application.js` link does not work for me on codetriage.com but `link application.js` does.

The error when a source isn't found should only show how to add that specific source for maximum clarity.

Author: schneems

lib/sprockets/rails/helper.rb

@@ -11,14 +11,10 @@ class AssetNotPrecompiled < StandardError
         def initialize(source)
           msg =
           if using_sprockets4?
-            "Asset `#{source}` was not declared to be precompiled in production.\n" +
-            "Declare links to your assets in `assets/config/manifest.js`.\n" +
-            "Examples:\n" +
-            "`//= link ../javascripts/application.js`\n" +
-            "`//= link_directory ../javascripts .js`\n" +
-            "`//= link_directory ../stylesheets .css`\n" +
-            "`//= link_tree ../javascripts .js`\n" +
-            "`//= link_tree ../images`\n"
+            "Asset `#{ source }` was not declared to be precompiled in production.\n" +
+            "Declare links to your assets in `app/assets/config/manifest.js`.\n\n" +
+            "  //= link #{ source }\n" +
+            "and restart your server"
           else
             "Asset was not declared to be precompiled in production.\n" +
             "Add `Rails.application.config.assets.precompile += " +

lib/sprockets/railtie.rb

@@ -66,6 +66,20 @@ module Sprockets
   class Railtie < ::Rails::Railtie
     include Sprockets::Rails::Utils
 
+    class ManifestNeededError < StandardError
+      def initialize
+        msg = "Expected to find a manifest file in `app/assets/config/manifest.js`\n" +
+        "But did not, please create this file and use it to link any assets that need\n" +
+        "to be rendered by your app:\n\n" +
+        "Example:\n" +
+        "  //= link_tree ../images\n"  +
+        "  //= link_directory ../javascripts .js\n" +
+        "  //= link_directory ../stylesheets .css\n"  +
+        "and restart your server"
+        super msg
+      end
+    end
+
     LOOSE_APP_ASSETS = lambda do |logical_path, filename|
         filename.start_with?(::Rails.root.join("app/assets").to_s) &&
         !['.js', '.css', ''].include?(File.extname(logical_path))
@@ -80,13 +94,19 @@ def configure(&block)
     config.assets = OrderedOptions.new
     config.assets._blocks     = []
     config.assets.paths       = []
+    config.assets.precompile  = []
     config.assets.prefix      = "/assets"
     config.assets.manifest    = nil
-    if using_sprockets4?
-      config.assets.precompile  = %w( manifest.js )
-    else
-      config.assets.precompile  = [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
+
+    initializer :set_default_precompile do |app|
+      if using_sprockets4?
+        raise ManifestNeededError if !::Rails.root.join("app/assets/config/manifest.js").exist?
+        app.config.assets.precompile  += %w( manifest.js )
+      else
+        app.config.assets.precompile  += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
+      end
     end
+
     config.assets.version     = ""
     config.assets.debug       = false
     config.assets.compile     = true

test/test_railtie.rb

@@ -38,6 +38,14 @@ def setup
     @app.config.middleware ||= Rails::Configuration::MiddlewareStackProxy.new
     @app.config.active_support.deprecation = :notify
     ActionView::Base # load ActionView
+
+    Dir.chdir(app.root) do
+      dir = "app/assets/config"
+      FileUtils.mkdir_p(dir)
+      File.open("#{ dir }/manifest.js", "w") do |f|
+        f << ""
+      end
+    end
   end
 
   def test_initialize
@@ -72,7 +80,7 @@ def test_defaults_to_compile_assets_with_env_and_manifest_available
     assert_equal ROOT, env.root
     assert_equal "", env.version
     assert env.cache
-    assert_equal [], env.paths
+    assert_equal ["#{ROOT}/app/assets/config"], env.paths
     assert_nil env.js_compressor
     assert_nil env.css_compressor
   end
@@ -120,7 +128,7 @@ def test_copies_paths
     app.initialize!
 
     assert env = app.assets
-    assert_equal ["#{ROOT}/javascripts", "#{ROOT}/stylesheets"],
+    assert_equal ["#{ROOT}/app/assets/config", "#{ROOT}/javascripts", "#{ROOT}/stylesheets"],
       env.paths.sort
   end
 
@@ -179,7 +187,7 @@ def test_configure
     app.initialize!
 
     assert env = app.assets
-    assert_equal ["#{ROOT}/javascripts", "#{ROOT}/stylesheets"],
+    assert_equal ["#{ROOT}/app/assets/config", "#{ROOT}/javascripts", "#{ROOT}/stylesheets"],
       env.paths.sort
   end
 
@@ -340,7 +348,7 @@ def test_direct_build_environment_call
     assert_kind_of Sprockets::Environment, env
 
     assert_equal ROOT, env.root
-    assert_equal ["#{ROOT}/javascripts", "#{ROOT}/stylesheets"],
+    assert_equal ["#{ROOT}/app/assets/config", "#{ROOT}/javascripts", "#{ROOT}/stylesheets"],
       env.paths.sort
   end
 end