Add support for Rails' recycable cache keys

This has been a feature from Rails 5.2 and is enabled by default
from Rails 6.0 and above.

app/models/alchemy/page/page_natures.rb

@@ -103,17 +103,17 @@ def layout_partial_name
         page_layout.parameterize.underscore
       end
 
-      # Returns the key that's taken for cache path.
+      # Returns the version that's taken for 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.
+      # If the page is the current preview it uses the +updated_at+ value as cache key.
       #
-      def cache_key
+      def cache_version
         if Page.current_preview == id
-          "alchemy/pages/#{id}-#{updated_at}"
+          updated_at.to_s
         else
-          "alchemy/pages/#{id}-#{published_at}"
+          published_at.to_s
         end
       end
 

spec/models/alchemy/page_spec.rb

@@ -735,15 +735,15 @@ class AnotherUrlPathClass; end
       end
     end
 
-    describe "#cache_key" do
+    describe "#cache_version" do
       let(:now) { Time.current }
       let(:last_week) { Time.current - 1.week }
 
       let(:page) do
         build_stubbed(:alchemy_page, updated_at: now, published_at: last_week)
       end
 
-      subject { page.cache_key }
+      subject { page.cache_version }
 
       before do
         expect(Page).to receive(:current_preview).and_return(preview)
@@ -753,15 +753,15 @@ class AnotherUrlPathClass; end
         let(:preview) { page.id }
 
         it "uses updated_at" do
-          is_expected.to eq("alchemy/pages/#{page.id}-#{now}")
+          is_expected.to eq(now.to_s)
         end
       end
 
       context "when current page not in preview mode" do
         let(:preview) { nil }
 
         it "uses published_at" do
-          is_expected.to eq("alchemy/pages/#{page.id}-#{last_week}")
+          is_expected.to eq(last_week.to_s)
         end
       end
     end