test: cover FormForHelperTest, persisted?

test/test_fragment_cache_viewer_independent.rb

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+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+require 'fileutils'
+
+# Session 62's "kierunki dalsze" debt, closed here: examples/blog/app/views/
+# posts/index.erb wraps its "recent posts" sidebar in `cache(:recent_posts_
+# sidebar, ...)` but does NOT include current_user in the cache key, unlike
+# the fresh_when etag on the same action (fixed in session 62, see
+# test_conditional_get_user_scoped_etag.rb). That asymmetry is deliberate,
+# not a leftover bug: the sidebar's own content (5 most recent post titles)
+# never depends on who is looking, and the parts of the page that DO vary
+# per viewer (signed-in-as banner, new-post form) render outside the cached
+# block entirely - so unlike the etag case, sharing one cache entry across
+# every viewer can never leak one visitor's per-user markup into another's
+# response. This test proves that reasoning against the exact template
+# shape (a cache block with viewer-independent content, sitting next to
+# viewer-dependent content rendered outside it), the same isolated-app
+# convention test_cache_helpers.rb and test_conditional_get_user_scoped_
+# etag.rb already use rather than loading examples/blog/app.rb directly.
+class FragmentCacheViewerIndependentTest < Minitest::Test
+  def setup
+    Prism::Cache.store.clear
+    @dir = Dir.mktmpdir('prism-fragment-cache-viewer')
+    FileUtils.mkdir_p(File.join(@dir, 'pages'))
+    @view = Prism::View.new(@dir)
+    write('pages/show.erb', <<~ERB)
+      <% cache(:recent_posts_sidebar,  expires_in: 60) do %>
+      <%= render_count! %> recent posts, none of them yours
+      <% end  %>
+      signed in as <%= viewer %>
+    ERB
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+    Prism::Cache.store.clear
+  end
+
+  def write(relative, source)
+    File.write(File.join(@dir, relative), source)
+  end
+
+  def render_as(viewer, tracker)
+    @view = Prism::View.new(@dir, helpers: Tracker.new(tracker))
+    @view.render('pages/show', locals: { viewer: viewer })
+  end
+
+  def test_cache_block_renders_once_and_is_shared_across_different_viewers
+    tracker = []
+
+    anon_output = render_as('anonymous', tracker)
+    alice_output = render_as('alice', tracker)
+    bob_output = render_as('bob', tracker)
+
+    assert_equal 1, tracker.size,
+                 'the fragment inside cache() should render exactly once total, not once per viewer'
+    assert_includes anon_output, '1 recent posts, none of them yours'
+    cached_line = anon_output.lines.find { |line| line.include?('recent posts') }
+    assert_equal cached_line, alice_output.lines.find { |line| line.include?('recent posts') }
+    assert_equal cached_line, bob_output.lines.find { |line| line.include?('recent posts') }
+  end
+
+  def test_content_outside_the_cache_block_still_varies_per_viewer
+    tracker = []
+
+    anon_output = render_as('anonymous', tracker)
+    alice_output = render_as('alice', tracker)
+
+    assert_includes anon_output, 'signed in as anonymous'
+    assert_includes alice_output, 'signed in as alice'
+    refute_equal anon_output, alice_output,
+                 'the viewer-dependent line must still differ even though the cached fragment is shared'
+  end
+
+  class Tracker
+    def initialize(calls)
+      @calls = calls
+    end
+
+    def render_count!
+      @calls << true
+      @calls.size
+    end
+  end
+end