Revert "Revert "PERF: Cache each theme field value once (#23192)" (#23354)" (#23356)

This reverts commit 9821ca9413.

spec/models/theme_spec.rb

@@ -1119,4 +1119,104 @@ HTML
       expect(messages).to include(["development-mode-theme-changed"])
     end
   end
+
+  describe "#lookup_field when a theme component is used in multiple themes" do
+    fab!(:theme_1) { Fabricate(:theme, user: user) }
+    fab!(:theme_2) { Fabricate(:theme, user: user) }
+    fab!(:child) { Fabricate(:theme, user: user, component: true) }
+
+    before_all do
+      theme_1.add_relative_theme!(:child, child)
+      theme_2.add_relative_theme!(:child, child)
+    end
+
+    it "efficiently caches fields of theme component by only caching the fields once across multiple themes" do
+      child.set_field(target: :common, name: "header", value: "World")
+      child.save!
+
+      expect(Theme.lookup_field(theme_1.id, :desktop, "header")).to eq("World")
+      expect(Theme.lookup_field(theme_2.id, :desktop, "header")).to eq("World")
+
+      expect(
+        Theme.cache.defer_get_set("#{child.id}:common:header:#{Theme.compiler_version}") { raise },
+      ).to eq(["World"])
+      expect(
+        Theme.cache.defer_get_set("#{child.id}:desktop:header:#{Theme.compiler_version}") { raise },
+      ).to eq(nil)
+
+      expect(
+        Theme
+          .cache
+          .defer_get_set("#{theme_1.id}:common:header:#{Theme.compiler_version}") { raise },
+      ).to eq(nil)
+      expect(
+        Theme
+          .cache
+          .defer_get_set("#{theme_1.id}:desktop:header:#{Theme.compiler_version}") { raise },
+      ).to eq(nil)
+
+      expect(
+        Theme
+          .cache
+          .defer_get_set("#{theme_2.id}:common:header:#{Theme.compiler_version}") { raise },
+      ).to eq(nil)
+      expect(
+        Theme
+          .cache
+          .defer_get_set("#{theme_2.id}:desktop:header:#{Theme.compiler_version}") { raise },
+      ).to eq(nil)
+    end
+
+    it "puts the parent value ahead of the child" do
+      theme_1.set_field(target: :common, name: "header", value: "theme_1")
+      theme_1.save!
+
+      child.set_field(target: :common, name: "header", value: "child")
+      child.save!
+
+      expect(Theme.lookup_field(theme_1.id, :desktop, "header")).to eq("theme_1\nchild")
+    end
+
+    it "puts parent translations ahead of child translations" do
+      theme_1.set_field(target: :translations, name: "en", value: <<~YAML)
+        en:
+          theme_1: "test"
+      YAML
+      theme_1.save!
+      theme_field = ThemeField.order(:id).last
+
+      child.set_field(target: :translations, name: "en", value: <<~YAML)
+        en:
+          child: "test"
+      YAML
+      child.save!
+      child_field = ThemeField.order(:id).last
+
+      expect(theme_field.value_baked).not_to eq(child_field.value_baked)
+      expect(Theme.lookup_field(theme_1.id, :translations, :en)).to eq(
+        [theme_field, child_field].map(&:value_baked).join("\n"),
+      )
+    end
+
+    it "prioritizes a locale over its fallback" do
+      theme_1.set_field(target: :translations, name: "en", value: <<~YAML)
+        en:
+          theme_1: "hello"
+      YAML
+      theme_1.save!
+      en_field = ThemeField.order(:id).last
+
+      theme_1.set_field(target: :translations, name: "es", value: <<~YAML)
+        es:
+          theme_1: "hola"
+      YAML
+      theme_1.save!
+      es_field = ThemeField.order(:id).last
+
+      expect(es_field.value_baked).not_to eq(en_field.value_baked)
+      expect(Theme.lookup_field(theme_1.id, :translations, :en)).to eq(en_field.value_baked)
+      expect(Theme.lookup_field(theme_1.id, :translations, :es)).to eq(es_field.value_baked)
+      expect(Theme.lookup_field(theme_1.id, :translations, :fr)).to eq(en_field.value_baked)
+    end
+  end
 end