[Fix #95] Raise When Dependencies Improperly Used

If a dependency is used in an ERB asset that references another asset, it will not be updated when the reference asset is updated. The fix is to use `//= depend_on` or its cousin `//= depend_on_asset` however this is easy to forget. See #95 for more information.

Currently Rails/Sprockets hides this problem, and only surfaces it when the app is deployed with precompilation to production multiple times. We know that you will have this problem if you are referencing assets from within other assets and not declaring them as dependencies. This PR checks if you've declared a given file as a dependency before including it via `asset_path`. If not a helpful error is raised:

```
Asset depends on 'bootstrap.js' to generate properly but has not declared the dependency
Please add: `//= depend_on_asset "bootstrap.js"` to '/Users/schneems/Documents/projects/codetriage/app/assets/javascripts/application.js.erb'
```

Implementation is quite simple and limited to `helper.rb`, additional code is all around tests.

ATP

Author: schneems

lib/sprockets/rails/helper.rb

@@ -5,6 +5,14 @@
 module Sprockets
   module Rails
     module Helper
+      class DependencyError <StandardError
+        def initialize(path, dep)
+          msg = "Asset depends on '#{dep}' to generate properly but has not declared the dependency\n"
+          msg << "Please add: `//= depend_on_asset \"#{dep}\"` to '#{path}'"
+          super msg
+        end
+      end
+
       if defined? ActionView::Helpers::AssetUrlHelper
         include ActionView::Helpers::AssetUrlHelper
         include ActionView::Helpers::AssetTagHelper
@@ -36,7 +44,14 @@ def self.extended(obj)
         end
       end
 
+      def check_dependencies!(dep)
+        return unless @_dependency_assets
+        return if     @_dependency_assets.detect { |asset| asset.include?(dep) }
+        raise DependencyError.new(self.pathname, dep)
+      end
+
       def compute_asset_path(path, options = {})
+        check_dependencies!(path)
         if digest_path = asset_digest_path(path)
           path = digest_path if digest_assets
           path += "?body=1" if options[:debug]

test/fixtures/error/dependency.js.erb

@@ -0,0 +1 @@
+<%= asset_path("bar.js") %>

test/fixtures/logo.png

@@ -1 +1,2 @@
+//= depend_on_asset "logo.png"
 p { background: url(<%= image_path "logo.png" %>); }

test/fixtures/url.css.erb

@@ -1 +1,2 @@
+//= depend_on_asset "foo.js"
 var url = '<%= javascript_path :foo %>';

test/fixtures/url.js.erb

@@ -21,7 +21,7 @@ def setup
     @view = ActionView::Base.new
     @view.extend ::Sprockets::Rails::Helper
     @view.assets_environment = @assets
-    @view.assets_prefix = "/assets"
+    @view.assets_prefix      = "/assets"
 
     # Rails 2.x
     unless @view.respond_to?(:config)
@@ -30,10 +30,11 @@ def setup
     end
 
     @assets.context_class.assets_prefix = @view.assets_prefix
-    @assets.context_class.config = @view.config
+    @assets.context_class.config        = @view.config
 
     @foo_js_digest  = @assets['foo.js'].digest
     @foo_css_digest = @assets['foo.css'].digest
+    @logo_digest    = @assets["logo.png"].digest
   end
 
   def test_truth
@@ -169,7 +170,7 @@ def test_stylesheet_path
 
   def test_asset_url
     assert_equal "var url = '//assets.example.com/assets/foo.js';\n", @assets["url.js"].to_s
-    assert_equal "p { background: url(//assets.example.com/images/logo.png); }\n", @assets["url.css"].to_s
+    assert_equal "p { background: url(//assets.example.com/assets/logo.png); }\n", @assets["url.css"].to_s
   end
 end
 
@@ -222,7 +223,7 @@ def test_asset_digest
 
   def test_asset_url
     assert_equal "var url = '/assets/foo.js';\n", @assets["url.js"].to_s
-    assert_equal "p { background: url(/images/logo.png); }\n", @assets["url.css"].to_s
+    assert_equal "p { background: url(/assets/logo.png); }\n", @assets["url.css"].to_s
   end
 end
 
@@ -279,7 +280,7 @@ def test_asset_digest_path
 
   def test_asset_url
     assert_equal "var url = '/assets/foo-#{@foo_js_digest}.js';\n", @assets["url.js"].to_s
-    assert_equal "p { background: url(/images/logo.png); }\n", @assets["url.css"].to_s
+    assert_equal "p { background: url(/assets/logo-#{@logo_digest}.png); }\n", @assets["url.css"].to_s
   end
 end
 
@@ -381,3 +382,11 @@ def test_asset_digest
     assert_equal @foo_css_digest, @view.asset_digest("foo.css")
   end
 end
+
+class ErrorsInHelpersTest < HelperTest
+  def test_dependency_error
+    assert_raise Sprockets::Rails::Helper::DependencyError do
+      @assets['error/dependency.js'].to_s
+    end
+  end
+end