No need to look up the notable separately in the ResolveService

app/services/discussions/resolve_service.rb

@@ -9,13 +9,7 @@ module Discussions
 
       discussion.resolve!(current_user)
 
-      notify_discussion_resolved(discussion)
-    end
-
-    def notify_discussion_resolved(discussion)
-      noteable = merge_request || discussion.noteable
-
-      MergeRequests::ResolvedDiscussionNotificationService.new(project, current_user).execute(noteable)
+      MergeRequests::ResolvedDiscussionNotificationService.new(project, current_user).execute(merge_request)
       SystemNoteService.discussion_continued_in_issue(discussion, project, current_user, follow_up_issue) if follow_up_issue
     end
 

spec/services/discussions/resolve_service_spec.rb

@@ -6,7 +6,7 @@ describe Discussions::ResolveService do
     let(:project) { merge_request.project }
     let(:merge_request) { discussion.noteable }
     let(:user) { create(:user) }
-    let(:service) { described_class.new(discussion.noteable.project, user) }
+    let(:service) { described_class.new(discussion.noteable.project, user, merge_request: merge_request) }
 
     before do
       project.team << [user, :master]