Fix Action View collection caching to store fragments as bare strings

actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb

@@ -89,15 +89,25 @@ def expanded_cache_key(key, view, template, digest_path)
       # If the partial is not already cached it will also be
       # written back to the underlying cache store.
       def fetch_or_cache_partial(cached_partials, template, order_by:)
-        order_by.index_with do |cache_key|
+        entries_to_write = {}
+
+        keyed_partials = order_by.index_with do |cache_key|
           if content = cached_partials[cache_key]
             build_rendered_template(content, template)
           else
-            yield.tap do |rendered_partial|
-              collection_cache.write(cache_key, rendered_partial.body)
+            rendered_partial = yield
+            if fragment = rendered_partial.body&.to_str
+              entries_to_write[cache_key] = fragment
             end
+            rendered_partial
           end
         end
+
+        unless entries_to_write.empty?
+          collection_cache.write_multi(entries_to_write)
+        end
+
+        keyed_partials
       end
   end
 end

actionview/test/activerecord/multifetch_cache_test.rb

@@ -30,6 +30,12 @@ class MultifetchCacheTest < ActiveRecordTestCase
 
   setup do
     Topic.update_all(updated_at: Time.now)
+    @cache_store_was = ActionView::PartialRenderer.collection_cache
+    ActionView::PartialRenderer.collection_cache = ActiveSupport::Cache::MemoryStore.new
+  end
+
+  teardown do
+    ActionView::PartialRenderer.collection_cache = @cache_store_was
   end
 
   def test_only_preloading_for_records_that_miss_the_cache
@@ -77,4 +83,19 @@ def test_preloads_all_records_if_using_cached_proc
     assert_equal first_req.first, second_req.first
     assert_includes second_req.last, %(WHERE "replies"."topic_id" IN (?, ?, ?))
   end
+
+  class InspectableStore < ActiveSupport::Cache::MemoryStore
+    attr_reader :data
+  end
+
+  def test_fragments_are_stored_as_bare_strings
+    cache = ActionView::PartialRenderer.collection_cache = InspectableStore.new
+    Topic.update_all(title: "title")
+    get :cached_true
+
+    assert_not_predicate cache.data, :empty?
+    cache.data.each_value do |entry|
+      assert_equal String, entry.value.class
+    end
+  end
 end