DEV: Preload translations when inline translation enabled

lib/discourse_translator/inline_translation.rb

@@ -65,6 +65,23 @@ def inject(plugin)
           !object.topic.locale_matches?(InlineTranslation.effective_locale) &&
           object.topic.translation_for(InlineTranslation.effective_locale).present?
       end
+
+      plugin.register_topic_preloader_associations(
+        {
+          association: :content_locale,
+          condition: -> do
+            SiteSetting.translator_enabled && SiteSetting.experimental_inline_translation
+          end,
+        },
+      )
+      plugin.register_topic_preloader_associations(
+        {
+          association: :translations,
+          condition: -> do
+            SiteSetting.translator_enabled && SiteSetting.experimental_inline_translation
+          end,
+        },
+      )
     end
   end
 end

spec/serializers/topic_list_serializer_spec.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+describe TopicListSerializer do
+  fab!(:topic1) { Fabricate(:topic) }
+  fab!(:topic2) { Fabricate(:topic) }
+  fab!(:topic3) { Fabricate(:topic) }
+  fab!(:user)
+
+  let(:topic_list) { TopicList.new(nil, user, [topic1, topic2, topic3]) }
+
+  before do
+    SiteSetting.translator_enabled = true
+    SiteSetting.experimental_inline_translation = true
+
+    # required for other core parts of topic_view
+    [topic1, topic2, topic3].each { |topic| topic.tap { |t| t.allowed_user_ids = [t.user_id] } }
+  end
+
+  describe "preloading" do
+    it "preloads locale and translations without N+1 queries" do
+      serializer = described_class.new(topic_list, scope: Guardian.new(user))
+
+      queries = track_sql_queries { serializer.as_json }
+
+      locale_queries = queries.count { |q| q.include?("discourse_translator_topic_locales") }
+      expect(locale_queries).to eq(1) # would be 3 if not preloaded
+
+      translation_queries =
+        queries.count { |q| q.include?("discourse_translator_topic_translations") }
+      expect(translation_queries).to eq(1) # would be 3 if not preloaded
+
+      expect(topic_list.topics.first.association(:content_locale)).to be_loaded
+    end
+
+    it "never preloads translations if SiteSetting.experimental_inline_translations is false" do
+      SiteSetting.experimental_inline_translation = false
+
+      serializer = described_class.new(topic_list, scope: Guardian.new(user))
+      queries = track_sql_queries { serializer.as_json }
+
+      locale_queries = queries.count { |q| q.include?("discourse_translator_topic_locales") }
+      expect(locale_queries).to eq(0)
+      translation_queries =
+        queries.count { |q| q.include?("discourse_translator_topic_translations") }
+      expect(translation_queries).to eq(0)
+    end
+  end
+end