DEV: Prevent N+1 on topic show and add index on post_locales for post_id (#314)

For privileged users in restrict_translation_by_group, we show the 🌐 per post. Showing the 🌐 depends on the post's locale. This commit ensures locale is preloaded to prevent N+1 and adds an index on post_locale for the post_id.

Author: nattsw

app/models/discourse_translator/post_locale.rb

@@ -21,3 +21,7 @@ class PostLocale < ActiveRecord::Base
 #  created_at      :datetime         not null
 #  updated_at      :datetime         not null
 #
+# Indexes
+#
+#  index_discourse_translator_post_locales_on_post_id  (post_id) UNIQUE
+#

db/migrate/20250630040849_add_index_post_locale_post_id.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class AddIndexPostLocalePostId < ActiveRecord::Migration[7.2]
+  disable_ddl_transaction!
+
+  def up
+    # clean up invalid index if index creation timeout
+    execute <<~SQL
+      DROP INDEX IF EXISTS index_discourse_translator_post_locales_on_post_id
+    SQL
+
+    execute <<~SQL
+      CREATE UNIQUE INDEX CONCURRENTLY index_discourse_translator_post_locales_on_post_id
+      ON discourse_translator_post_locales (post_id)
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

plugin.rb

@@ -31,6 +31,10 @@ module ::DiscourseTranslator
     Topic.prepend(DiscourseTranslator::Extensions::TopicExtension)
   end
 
+  TopicView.on_preload do |topic_view|
+    topic_view.instance_variable_set(:@posts, topic_view.posts.includes(:content_locale))
+  end
+
   add_to_serializer :post, :can_translate do
     scope.can_translate?(object)
   end