Keep the URL of live pages stable when the title changes

app/models/spina/page.rb

@@ -85,6 +85,7 @@ def next_sibling
     end
 
     def set_materialized_path
+      freeze_url_title
       self.old_path = materialized_path
       self.materialized_path = localized_materialized_path
 
@@ -118,6 +119,17 @@ def rewrite_rule
       RewriteRule.where(old_path: old_path).first_or_create.update(new_path: materialized_path) if old_path != materialized_path
     end
 
+    # Locks the slug of a live page by writing it to url_title, so changing
+    # the title no longer changes the URL. The URL can still be changed
+    # explicitly through url_title.
+    def freeze_url_title
+      return unless persisted? && live?
+      return if homepage? || url_title(default: nil).present?
+
+      current_slug = materialized_path.to_s.split("/").last
+      self.url_title = current_slug if current_slug.present?
+    end
+
     def localized_materialized_path
       segments = if Mobility.locale == I18n.default_locale
         [Spina.mounted_at, generate_materialized_path]

test/models/spina/page_test.rb

@@ -40,6 +40,29 @@ def setup
       assert_equal "/custom-slug", @demo.materialized_path
     end
 
+    test "changing the title of a live page keeps the url" do
+      page = FactoryBot.create(:page, title: "First title")
+      page.update(title: "Second title")
+
+      assert_equal "/first-title", page.materialized_path
+      assert_empty RewriteRule.all
+    end
+
+    test "changing url_title still changes the url" do
+      page = FactoryBot.create(:page, title: "First title")
+      page.update(url_title: "different-url")
+
+      assert_equal "/different-url", page.materialized_path
+      assert_equal [["/first-title", "/different-url"]], RewriteRule.pluck(:old_path, :new_path)
+    end
+
+    test "drafts keep following the title" do
+      page = FactoryBot.create(:page, title: "First title", draft: true)
+      page.update(title: "Second title")
+
+      assert_equal "/second-title", page.materialized_path
+    end
+
     test "build slug from ancestors" do
       about = FactoryBot.create :about_page
       page = FactoryBot.create :services_page