Properly cleaning up memoized values

app/models/concerns/resolvable_discussion.rb

@@ -44,7 +44,9 @@ module ResolvableDiscussion
   end
 
   def first_note
-    notes.first
+    strong_memoize(:first_note) do
+      notes.first
+    end
   end
 
   def first_note_to_resolve
@@ -102,8 +104,8 @@ module ResolvableDiscussion
     # Set the notes array to the updated notes
     @notes = notes_relation.fresh.to_a # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
-    self.class.memoized_values.each do |var|
-      instance_variable_set(:"@#{var}", nil)
+    self.class.memoized_values.each do |name|
+      clear_memoization(name)
     end
   end
 end