PERF: Allow user serializer to make use of preloaded custom fields (#9074)

app/models/concerns/has_custom_fields.rb

@@ -142,6 +142,10 @@ module HasCustomFields
     super
   end
 
+  def custom_fields_preloaded?
+    !!@preloaded_custom_fields
+  end
+
   def custom_field_preloaded?(name)
     @preloaded_custom_fields && @preloaded_custom_fields.key?(name)
   end

app/serializers/user_card_serializer.rb

@@ -149,14 +149,14 @@ class UserCardSerializer < BasicUserSerializer
   end
 
   def custom_fields
-    fields = User.whitelisted_user_custom_fields(scope)
+    fields = custom_field_keys
-
-    if scope.can_edit?(object)
-      fields += DiscoursePluginRegistry.serialized_current_user_fields.to_a
-    end
 
     if fields.present?
-      User.custom_fields_for_ids([object.id], fields)[object.id] || {}
+      if object.custom_fields_preloaded?
+        {}.tap { |h| fields.each { |f| h[f] = object.custom_fields[f] } }
+      else
+        User.custom_fields_for_ids([object.id], fields)[object.id] || {}
+      end
     else
       {}
     end
@@ -197,4 +197,11 @@ class UserCardSerializer < BasicUserSerializer
   def card_background_upload_url
     object.card_background_upload&.url
   end
+
+  private
+
+  def custom_field_keys
+    # Can be extended by other serializers
+    User.whitelisted_user_custom_fields(scope)
+  end
 end

app/serializers/user_serializer.rb

@@ -270,4 +270,16 @@ class UserSerializer < UserCardSerializer
     object.profile_background_upload&.url
   end
 
+  private
+
+  def custom_field_keys
+    fields = super
+
+    if scope.can_edit?(object)
+      fields += DiscoursePluginRegistry.serialized_current_user_fields.to_a
+    end
+
+    fields
+  end
+
 end