Enforce fresh ETag header after collection changes

Add ActiveRecord::Relation#cache_key_with_version. This method will be
used by ActionController::ConditionalGet to ensure that when collection
cache versioning is enabled, requests using ConditionalGet don't return
the same ETag header after a collection is modified.

Prior to the introduction of collection cache versioning in
4f2ac80d4c, all collection cache keys
included a version. However, with cache versioning enabled, collection
cache keys remain constant. In turn, ETag headers remain constant,
rendering them ineffective.

This commit takes the cache_key_with_version method used for individual
Active Record objects (from aa8749eb52),
and adds it to collections.

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   Enforce fresh ETag header after a collection's contents change by adding
+    ActiveRecord::Relation#cache_key_with_version. This method will be used by
+    ActionController::ConditionalGet to ensure that when collection cache versioning
+    is enabled, requests using ConditionalGet don't return the same ETag header
+    after a collection is modified. Fixes #38078.
+
+    *Aaron Lipman*
+
 *   Skip test database when running `db:create` or `db:drop` in development
     with `DATABASE_URL` set.
 

activerecord/lib/active_record/relation.rb

@@ -385,6 +385,15 @@ module ActiveRecord
     end
     private :compute_cache_version
 
+    # Returns a cache key along with the version.
+    def cache_key_with_version
+      if version = cache_version
+        "#{cache_key}-#{version}"
+      else
+        cache_key
+      end
+    end
+
     # Scope all queries to the current scope.
     #
     #   Comment.where(post_id: 1).scoping do

activerecord/test/cases/collection_cache_key_test.rb

@@ -198,6 +198,16 @@ module ActiveRecord
       end
     end
 
+    test "cache_key_with_version contains key and version regardless of collection_cache_versioning setting" do
+      key_with_version_1 = Developer.all.cache_key_with_version
+      assert_match(/\Adevelopers\/query-(\h+)-(\d+)-(\d+)\z/, key_with_version_1)
+
+      with_collection_cache_versioning do
+        key_with_version_2 = Developer.all.cache_key_with_version
+        assert_equal(key_with_version_1, key_with_version_2)
+      end
+    end
+
     def with_collection_cache_versioning(value = true)
       @old_collection_cache_versioning = ActiveRecord::Base.collection_cache_versioning
       ActiveRecord::Base.collection_cache_versioning = value