3 files changed, 20 insertions, 4 deletions

diff --git a/app/helpers/admin_helper.rb b/app/helpers/admin_helper.rb
index 08336ef..1e84620 100644
--- a/app/helpers/admin_helper.rb
+++ b/app/helpers/admin_helper.rb
@@ -8,4 +8,10 @@ module AdminHelper
       link_to raw('<span class="inactive">English</span>'), url_for(params.permit(:locale, :page_path).to_h.merge('locale' => 'de'))
     end
   end
+
+  def mtime_busted_path(path)
+    file = Rails.public_path.join(path.sub(%r{\A/}, ""))
+    raise "Static asset not found for cache-busting: #{path} (looked for #{file})" unless File.exist?(file)
+    "#{path}?v=#{File.mtime(file).to_i}"
+  end
 end
diff --git a/app/views/layouts/admin.html.erb b/app/views/layouts/admin.html.erb
index 8bbc0d5..0856a0f 100644
--- a/app/views/layouts/admin.html.erb
+++ b/app/views/layouts/admin.html.erb
@@ -10,10 +10,10 @@
     <%= javascript_tag "var AUTH_TOKEN = #{form_authenticity_token.inspect};" if protect_against_forgery? %>
     <%= javascript_include_tag 'admin_bundle' %>
     <%= tinymce_assets %>
-    <link rel="stylesheet" href="/stylesheets/admin.css">
-    <script src="/javascripts/admin_search.js"></script>
-    <script src="/javascripts/admin_interface.js"></script>
-    <script src="/javascripts/related_assets.js"></script>
+    <link rel="stylesheet" href="<%= mtime_busted_path('/stylesheets/admin.css') %>">
+    <script src="<%= mtime_busted_path('/javascripts/admin_search.js') %>"></script>
+    <script src="<%= mtime_busted_path('/javascripts/admin_interface.js') %>"></script>
+    <script src="<%= mtime_busted_path('/javascripts/related_assets.js') %>"></script>
     <script>
       var ADMIN_SEARCH_URL = "<%= admin_search_path %>";
       var ADMIN_MENU_SEARCH_URL = "<%= admin_menu_search_path %>";
diff --git a/test/models/helpers/admin_helper_test.rb b/test/models/helpers/admin_helper_test.rb
index 23d9f40..94e9861 100644
--- a/test/models/helpers/admin_helper_test.rb
+++ b/test/models/helpers/admin_helper_test.rb
@@ -1,4 +1,14 @@
 require 'test_helper'
 
 class AdminHelperTest < ActionView::TestCase
+  test "mtime_busted_path appends the file's real mtime as a query param" do
+    path = "/stylesheets/admin.css"
+    expected_mtime = File.mtime(Rails.public_path.join("stylesheets/admin.css")).to_i
+
+    assert_equal "#{path}?v=#{expected_mtime}", mtime_busted_path(path)
+  end
+
+  test "mtime_busted_path raises clearly for a missing file rather than silently omitting the version" do
+    assert_raises(RuntimeError) { mtime_busted_path("/stylesheets/does_not_exist.css") }
+  end
 end