FIX: N+1 in admin themes page (#32763)

`strict_loading` was added to prevent it happening in the future. Few
adjustments had to be made:
- include color_scheme and color_scheme_colors, also for parent and
child themes;
- internal translations were using preload_fields, but it was too deep
to correctly use preloaded tables. I had to pass
`preloaded_locale_fields` manually;
- include theme in color_scheme.

Before:
<img width="663" alt="Screenshot 2025-05-15 at 3 43 47 pm"
src="https://github.com/user-attachments/assets/b55ce11e-80cb-43eb-8e31-940b0e9859f3"
/>

After:
<img width="665" alt="Screenshot 2025-05-16 at 11 29 00 am"
src="https://github.com/user-attachments/assets/f00bac19-f64b-4048-b220-4d0a9d90a929"
/>

app/controllers/admin/themes_controller.rb

@@ -169,8 +169,10 @@ class Admin::ThemesController < Admin::AdminController
   end
 
   def index
-    @themes = Theme.include_relations.order(:name)
+    @themes = Theme.strict_loading.include_relations.order(:name)
-    @color_schemes = ColorScheme.all.includes(:theme, color_scheme_colors: :color_scheme).to_a
+
+    @color_schemes =
+      ColorScheme.strict_loading.all.includes(:theme, color_scheme_colors: :color_scheme).to_a
 
     payload = {
       themes: serialize_data(@themes, ThemeSerializer),

app/models/theme.rb

@@ -84,11 +84,11 @@ class Theme < ActiveRecord::Base
   scope :include_relations,
         -> do
           include_basic_relations.includes(
-            :child_themes,
             :theme_settings,
             :settings_field,
-            :color_scheme,
+            color_scheme: %i[color_scheme_colors],
             theme_fields: %i[upload theme_settings_migration],
+            child_themes: %i[color_scheme locale_fields theme_translation_overrides],
           )
         end
 
@@ -99,7 +99,8 @@ class Theme < ActiveRecord::Base
             :user,
             :locale_fields,
             :theme_translation_overrides,
-            parent_themes: %i[locale_fields theme_translation_overrides],
+            color_scheme: %i[theme],
+            parent_themes: %i[color_scheme locale_fields theme_translation_overrides],
           )
         end
 
@@ -658,15 +659,16 @@ class Theme < ActiveRecord::Base
     end
   end
 
-  def internal_translations
+  def internal_translations(preloaded_locale_fields: nil)
-    @internal_translations ||= translations(internal: true)
+    @internal_translations ||=
+      translations(internal: true, preloaded_locale_fields: preloaded_locale_fields)
   end
 
-  def translations(internal: false)
+  def translations(internal: false, preloaded_locale_fields: nil)
     fallbacks = I18n.fallbacks[I18n.locale]
     begin
       data =
-        locale_fields.first&.translation_data(
+        (preloaded_locale_fields&.first || locale_fields.first)&.translation_data(
           with_overrides: false,
           internal: internal,
           fallback_fields: locale_fields,

app/serializers/basic_theme_serializer.rb

@@ -12,6 +12,9 @@ class BasicThemeSerializer < ApplicationSerializer
   end
 
   def description
-    object.internal_translations.find { |t| t.key == "theme_metadata.description" }&.value
+    object
+      .internal_translations(preloaded_locale_fields: object.locale_fields)
+      .find { |t| t.key == "theme_metadata.description" }
+      &.value
   end
 end

app/serializers/color_scheme_serializer.rb

@@ -8,10 +8,6 @@ class ColorSchemeSerializer < ApplicationSerializer
     object.theme&.name
   end
 
-  def theme_id
-    object.theme&.id
-  end
-
   def colors
     db_colors = object.colors.index_by(&:name)
     object.resolved_colors.map do |name, default|

spec/requests/admin/themes_controller_spec.rb

@@ -476,6 +476,44 @@ RSpec.describe Admin::ThemesController do
 
         expect(theme_json["remote_theme"]["remote_version"]).to eq("7")
       end
+
+      it "does not result in N+1 queries" do
+        # warmup
+        get "/admin/themes.json"
+        expect(response.status).to eq(200)
+
+        theme = Fabricate(:theme, color_scheme: Fabricate(:color_scheme))
+        Fabricate(
+          :theme_field,
+          target_id: Theme.targets[:translations],
+          theme: theme,
+          name: "en",
+          value:
+            "en:\n  theme_metadata:\n    description: \"A simple, beautiful theme that improves the out of the box experience for Discourse sites.\"\n  topic_pinned: \"Pinned\"\n  topic_hot: \"Hot\"\n  user_replied: \"replied\"\n  user_posted: \"posted\"\n  user_updated: \"updated\"\n",
+        )
+        first_request_queries =
+          track_sql_queries do
+            get "/admin/themes.json"
+            expect(response.status).to eq(200)
+          end
+
+        theme_2 = Fabricate(:theme, color_scheme: Fabricate(:color_scheme))
+        Fabricate(
+          :theme_field,
+          target_id: Theme.targets[:translations],
+          theme: theme_2,
+          name: "en",
+          value:
+            "en:\n  theme_metadata:\n    description: \"A simple, beautiful theme that improves the out of the box experience for Discourse sites.\"\n  topic_pinned: \"Pinned\"\n  topic_hot: \"Hot\"\n  user_replied: \"replied\"\n  user_posted: \"posted\"\n  user_updated: \"updated\"\n",
+        )
+        second_request_queries =
+          track_sql_queries do
+            get "/admin/themes.json"
+            expect(response.status).to eq(200)
+          end
+
+        expect(first_request_queries.count).to eq(second_request_queries.count)
+      end
     end
 
     it "allows themes and components to be edited" do