DEV: Serialize categories in topic lists (#23597)

At this moment, this feature is under a site setting named
lazy_load_categories.

In the future, categories will no longer be preloaded through site data.
This commit add information about categories in topic list and ensures
that data is used to display topic list items.

Parent categories are serialized too because they are necessary to
render {{category-link}}.

app/assets/javascripts/discourse/app/models/topic-list.js

@@ -5,6 +5,7 @@ import { isEmpty } from "@ember/utils";
 import { Promise } from "rsvp";
 import { ajax } from "discourse/lib/ajax";
 import RestModel from "discourse/models/rest";
+import Site from "discourse/models/site";
 import User from "discourse/models/user";
 import deprecated from "discourse-common/lib/deprecated";
 import { getOwnerWithFallback } from "discourse-common/lib/get-owner";
@@ -168,6 +169,12 @@ TopicList.reopenClass({
     const users = extractByKey(result.users, User);
     const groups = extractByKey(result.primary_groups, EmberObject);
 
+    if (result.topic_list.categories) {
+      result.topic_list.categories.forEach((c) => {
+        Site.current().updateCategory(c);
+      });
+    }
+
     return result.topic_list[listKey].map((t) => {
       t.posters.forEach((p) => {
         p.user = users[p.user_id];

app/models/topic_list.rb

@@ -76,6 +76,10 @@ class TopicList
     @topics ||= load_topics
   end
 
+  def categories
+    @categories ||= topics.map { |t| [t.category, t.category.parent_category] }.uniq.flatten.compact
+  end
+
   def load_topics
     @topics = @topics_input
 
@@ -128,7 +132,12 @@ class TopicList
       ft.topic_list = self
     end
 
-    topic_preloader_associations = [:image_upload, { topic_thumbnails: :optimized_image }]
+    topic_preloader_associations = [
+      :image_upload,
+      { topic_thumbnails: :optimized_image },
+      { category: :parent_category },
+    ]
+
     topic_preloader_associations.concat(DiscoursePluginRegistry.topic_preloader_associations.to_a)
 
     ActiveRecord::Associations::Preloader.new(

app/serializers/topic_category_serializer.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+class TopicCategorySerializer < ApplicationSerializer
+  attributes :id,
+             :name,
+             :color,
+             :text_color,
+             :slug,
+             :description_text,
+             :read_restricted,
+             :parent_category_id
+
+  def include_parent_category_id?
+    parent_category_id
+  end
+
+  def name
+    if object.uncategorized?
+      I18n.t("uncategorized_category_name", locale: SiteSetting.default_locale)
+    else
+      object.name
+    end
+  end
+
+  def description_text
+    if object.uncategorized?
+      I18n.t("category.uncategorized_description", locale: SiteSetting.default_locale)
+    else
+      object.description_text
+    end
+  end
+end

app/serializers/topic_list_serializer.rb

@@ -12,6 +12,7 @@ class TopicListSerializer < ApplicationSerializer
   has_many :topics, serializer: TopicListItemSerializer, embed: :objects
   has_many :shared_drafts, serializer: TopicListItemSerializer, embed: :objects
   has_many :tags, serializer: TagSerializer, embed: :objects
+  has_many :categories, serializer: TopicCategorySerializer, embed: :objects
 
   def can_create_topic
     scope.can_create?(Topic)
@@ -36,4 +37,8 @@ class TopicListSerializer < ApplicationSerializer
   def include_tags?
     SiteSetting.tagging_enabled && object.tags.present?
   end
+
+  def include_categories?
+    SiteSetting.lazy_load_categories
+  end
 end

config/site_settings.yml

@@ -2174,6 +2174,9 @@ developer:
   instrument_gc_stat_per_request:
     default: false
     hidden: true
+  lazy_load_categories:
+    default: false
+    hidden: true
 
 navigation:
   navigation_menu:

spec/requests/list_controller_spec.rb

@@ -223,6 +223,38 @@ RSpec.describe ListController do
         expect(response.body).not_to include(restricted_tag.name)
       end
     end
+
+    context "with lazy_load_categories" do
+      fab!(:category) { Fabricate(:category) }
+      fab!(:subcategory) { Fabricate(:category, parent_category: category) }
+
+      before { topic.update!(category: subcategory) }
+
+      it "returns categories and parent categories if true" do
+        SiteSetting.lazy_load_categories = true
+
+        get "/latest.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["topic_list"]["topics"].length).to eq(1)
+        expect(response.parsed_body["topic_list"]["topics"][0]["id"]).to eq(topic.id)
+        expect(response.parsed_body["topic_list"]["categories"].length).to eq(2)
+        expect(
+          response.parsed_body["topic_list"]["categories"].map { |c| c["id"] },
+        ).to contain_exactly(category.id, subcategory.id)
+      end
+
+      it "does not return categories if not true" do
+        SiteSetting.lazy_load_categories = false
+
+        get "/latest.json"
+
+        expect(response.status).to eq(200)
+        expect(response.parsed_body["topic_list"]["topics"].length).to eq(1)
+        expect(response.parsed_body["topic_list"]["topics"][0]["id"]).to eq(topic.id)
+        expect(response.parsed_body["topic_list"]["categories"]).to eq(nil)
+      end
+    end
   end
 
   describe "categories and X" do