Prevent n+1 when loading topic post's translation metadata

Author: nattsw

lib/discourse_translator/topic_view_serializer_extension.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module DiscourseTranslator
+  module TopicViewSerializerExtension
+    def posts
+      if SiteSetting.translator_enabled?
+        object.instance_variable_set(:@posts, object.posts.includes(:content_locale))
+      end
+      super
+    end
+  end
+end

plugin.rb

@@ -31,6 +31,7 @@ module ::DiscourseTranslator
     Guardian.prepend(DiscourseTranslator::GuardianExtension)
     Post.prepend(DiscourseTranslator::PostExtension)
     Topic.prepend(DiscourseTranslator::TopicExtension)
+    TopicViewSerializer.prepend(DiscourseTranslator::TopicViewSerializerExtension)
   end
 
   on(:post_process_cooked) do |_, post|

spec/serializers/topic_view_serializer_spec.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe TopicViewSerializer do
+  fab!(:user)
+  fab!(:topic)
+  fab!(:post1) { Fabricate(:post, topic: topic).set_detected_locale("en") }
+  fab!(:post2) { Fabricate(:post, topic: topic).set_detected_locale("es") }
+  fab!(:post3) { Fabricate(:post, topic: topic).set_detected_locale("ja") }
+
+  before do
+    SiteSetting.translator_enabled = true
+    SiteSetting.restrict_translation_by_group = "#{Group::AUTO_GROUPS[:everyone]}"
+    SiteSetting.restrict_translation_by_poster_group = "#{Group::AUTO_GROUPS[:everyone]}"
+  end
+
+  it "preloads translations without N+1 queries" do
+    topic_view = TopicView.new(topic)
+    serializer = TopicViewSerializer.new(topic_view, scope: Guardian.new(user), root: false)
+
+    # ensure translation data is included in the JSON
+    json = {}
+    queries = track_sql_queries { json = serializer.as_json }
+    posts_json = json[:post_stream][:posts]
+    expect(posts_json.map { |p| p[:can_translate] }).to eq([false, true, true])
+
+    translation_queries = queries.count { |q| q.include?("discourse_translator_post_locales") }
+    expect(translation_queries).to eq(1) # would be 3 (posts) if not preloaded
+
+    expect(topic_view.posts.first.association(:content_locale)).to be_loaded
+  end
+end