Use page version's updated_at timestamp as cache key

Currently, it's hard for us to invalidate caches for Alchemy pages when
content that's referenced through ingredients with related objects
changes.

For example, in a situation where a user combines Alchemy and Solidus
using the `alchemy_solidus` gem, a page's cache key does not update when
a product that's referenced through a SpreeProduct ingredient changes.

There's a PR up on the alchemy_solidus gem that touches ingredients in
these situations, and with this change, that touching can be used for
breaking caches. [1]

In the unlikely event the requested version is not available, we use the
page's updated_at timestamp as a fallback.

[1](AlchemyCMS/alchemy-solidus#108)

app/models/alchemy/page/page_natures.rb

@@ -101,18 +101,15 @@ def layout_partial_name
         page_layout.parameterize.underscore
       end
 
-      # Returns the version that's taken for Rails' recycable cache key.
+      # Returns the string version that's used as Rails' recycable cache key.
       #
-      # Uses the +published_at+ value that's updated when the user publishes the page.
-      #
-      # If the page is the current preview it uses the +updated_at+ value as cache key.
+      # In preview mode, it will take the draft version's updated_at timestamp.
+      # In public mode, it will take the public version's updated_at timestamp.
+      # If no version is available, it will take the page's updated_at timestamp.
       #
       def cache_version
-        if Current.preview_page == self
-          updated_at.to_s
-        else
-          published_at.to_s
-        end
+        relevant_page_version = (Current.preview_page == self) ? draft_version : public_version
+        (relevant_page_version&.updated_at || updated_at).to_s
       end
 
       # Returns true if the page cache control headers should be set.

spec/models/alchemy/page_spec.rb

@@ -705,12 +705,16 @@ module Alchemy
 
     describe "#cache_version" do
       let(:now) { Time.current }
+      let(:yesterday) { Time.current - 1.day }
       let(:last_week) { Time.current - 1.week }
 
       let(:page) do
-        build_stubbed(:alchemy_page, updated_at: now, published_at: last_week)
+        build_stubbed(:alchemy_page, public_version: public_version, draft_version: draft_version, updated_at: yesterday)
       end
 
+      let(:public_version) { build_stubbed(:alchemy_page_version, updated_at: last_week) }
+      let(:draft_version) { build_stubbed(:alchemy_page_version, updated_at: now) }
+
       subject { page.cache_version }
 
       before do
@@ -732,6 +736,34 @@ module Alchemy
           is_expected.to eq(last_week.to_s)
         end
       end
+
+      context "if page has no public version" do
+        let(:public_version) { nil }
+
+        context "in preview mode" do
+          let(:preview) { page }
+
+          it "uses updated_at" do
+            is_expected.to eq(now.to_s)
+          end
+
+          context "if page has no draft version" do
+            let(:draft_version) { nil }
+
+            it "returns nil" do
+              is_expected.to eq(yesterday.to_s)
+            end
+          end
+        end
+
+        context "not in preview mode" do
+          let(:preview) { nil }
+
+          it "uses updated_at" do
+            is_expected.to eq(yesterday.to_s)
+          end
+        end
+      end
     end
 
     describe "#public_version" do