DEV: Add category custom field preloading to CategoryList (#23969)

This commit also introduced a plugin API for preloading category custom
fields.

app/models/category_list.rb

@@ -6,6 +6,9 @@ class CategoryList
   cattr_accessor :preloaded_topic_custom_fields
   self.preloaded_topic_custom_fields = Set.new
 
+  cattr_accessor :preloaded_category_custom_fields
+  self.preloaded_category_custom_fields = Set.new
+
   attr_accessor :categories, :uncategorized
 
   def self.register_included_association(association)
@@ -139,6 +142,10 @@ class CategoryList
 
     @categories = query.to_a
 
+    if preloaded_category_custom_fields.any?
+      Category.preload_custom_fields(@categories, preloaded_category_custom_fields)
+    end
+
     include_subcategories = @options[:include_subcategories] == true
 
     notification_levels = CategoryUser.notification_levels_for(@guardian.user)

lib/plugin/instance.rb

@@ -290,6 +290,13 @@ class Plugin::Instance
     Upload.add_in_use_callback(&block)
   end
 
+  # Registers a category custom field to be loaded when rendering a category list
+  # Example usage:
+  #   register_category_list_preloaded_category_custom_fields("custom_field")
+  def register_category_list_preloaded_category_custom_fields(field)
+    CategoryList.preloaded_category_custom_fields << field
+  end
+
   def custom_avatar_column(column)
     reloadable_patch do |plugin|
       UserLookup.lookup_columns << column

spec/models/category_list_spec.rb

@@ -357,4 +357,21 @@ RSpec.describe CategoryList do
       DiscoursePluginRegistry.clear_modifiers!
     end
   end
+
+  describe "with custom fields" do
+    fab!(:category) { Fabricate(:category, user: admin) }
+
+    before { category.upsert_custom_fields("bob" => "marley") }
+    after { CategoryList.preloaded_category_custom_fields = Set.new }
+
+    it "can preloads custom fields" do
+      CategoryList.preloaded_category_custom_fields << "bob"
+
+      expect(category_list.categories[-1].custom_field_preloaded?("bob")).to eq(true)
+    end
+
+    it "does not preload fields that were not set for preloading" do
+      expect(category_list.categories[-1].custom_field_preloaded?("bob")).to be_falsey
+    end
+  end
 end

spec/requests/categories_controller_spec.rb

@@ -372,6 +372,44 @@ RSpec.describe CategoriesController do
         response.parsed_body["category_list"]["categories"].map { |x| x["id"] },
       ).not_to include(uncategorized.id)
     end
+
+    describe "with serialized category custom fields" do
+      class CategoryPlugin < Plugin::Instance
+      end
+
+      let!(:plugin) do
+        plugin = CategoryPlugin.new
+        plugin.add_to_serializer(:basic_category, :bob) { object.custom_fields["bob"] }
+        plugin
+      end
+
+      before { category.upsert_custom_fields("bob" => "marley") }
+      after { CategoryList.preloaded_category_custom_fields = Set.new }
+
+      context "when custom fields are not preloaded" do
+        it "increases the query count" do
+          queries = track_sql_queries { get "/categories.json" }
+
+          expect(response.status).to eq(200)
+          category = response.parsed_body["category_list"]["categories"][-1]
+          expect(category["bob"]).to eq("marley")
+          expect(queries.count).to eq(7)
+        end
+      end
+
+      context "when custom fields are preloaded" do
+        before { CategoryList.preloaded_category_custom_fields << "bob" }
+
+        it "does not increase the query count" do
+          queries = track_sql_queries { get "/categories.json" }
+
+          expect(response.status).to eq(200)
+          category = response.parsed_body["category_list"]["categories"][-1]
+          expect(category["bob"]).to eq("marley")
+          expect(queries.count).to eq(6)
+        end
+      end
+    end
   end
 
   describe "extensibility event" do