Use last-modified-at in PagesController

app/controllers/alchemy/pages_controller.rb

@@ -216,7 +216,7 @@ def page_etag
     def render_fresh_page?
       must_not_cache? || stale?(
         etag: page_etag,
-        last_modified: @page.published_at,
+        last_modified: @page.last_modified_at,
         public: !@page.restricted,
         template: "pages/show"
       )

app/models/alchemy/page/page_natures.rb

@@ -101,19 +101,18 @@ def layout_partial_name
         page_layout.parameterize.underscore
       end
 
-      # Returns the string version of the +last_modified_at+ timestamp.
+      # Returns the version string that's taken for Rails' recycable cache key.
       #
       def cache_version
         last_modified_at&.to_s
       end
 
-      # Returns the timestamp that the page was last modified at, regardless of through.
+      # Returns the timestamp that the page was last modified at, regardless of through
       # publishing or editing page, or through a change of related objects through ingredients.
       # Respects the public version not changing if editing a preview.
       #
       # 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 last_modified_at
         relevant_page_version = (Current.preview_page == self) ? draft_version : public_version

spec/requests/alchemy/page_request_caching_spec.rb

@@ -3,8 +3,7 @@
 require "rails_helper"
 
 RSpec.describe "Page request caching" do
-  let!(:page) { create(:alchemy_page, :public, published_at: published_at) }
-  let(:published_at) { nil }
+  let!(:page) { create(:alchemy_page, :public) }
 
   context "when caching is disabled in app" do
     before do
@@ -81,18 +80,17 @@
         expect(response.headers).to have_key("ETag")
       end
 
-      it "does not set last-modified header" do
-        get "/#{page.urlname}"
-        expect(response.headers).to_not have_key("Last-Modified")
-      end
+      context "and public version is present" do
+        let(:jan_first) { Time.new(2020, 1, 1) }
 
-      context "and published_at is set" do
-        let(:published_at) { DateTime.yesterday }
+        before do
+          allow_any_instance_of(Alchemy::Page).to receive(:last_modified_at) { jan_first }
+        end
 
         it "sets last-modified header" do
           get "/#{page.urlname}"
           expect(response.headers).to have_key("Last-Modified")
-          expect(response.headers["Last-Modified"]).to eq(published_at.httpdate)
+          expect(response.headers["Last-Modified"]).to eq(jan_first.httpdate)
         end
       end
     end