unique quiz constraint

internal/service/quiz_service.go

@@ -199,6 +199,9 @@ func (s *QuizService) Create(ctx context.Context, actorID string, d QuizDraft) (
 	if err != nil {
 		return nil, err
 	}
+	if err := s.ensureTargetFree(ctx, in, ""); err != nil {
+		return nil, err
+	}
 	id, err := s.quizzes.Create(ctx, in, actorID)
 	if err != nil {
 		return nil, err
@@ -223,12 +226,34 @@ func (s *QuizService) Update(ctx context.Context, id string, d QuizDraft) (*mode
 	if err != nil {
 		return nil, err
 	}
+	if err := s.ensureTargetFree(ctx, in, id); err != nil {
+		return nil, err
+	}
 	if err := s.quizzes.Update(ctx, id, in, !locked); err != nil {
 		return nil, err
 	}
 	return s.GetAdmin(ctx, id)
 }
 
+// ensureTargetFree refuses a second quiz on the same chapter, lecture or
+// conversation, naming the one already there so the admin can edit it
+// instead. Stand-alone quizzes are never limited. The unique index backs
+// this up against a race between two saves.
+func (s *QuizService) ensureTargetFree(ctx context.Context, in repository.QuizInput, selfID string) error {
+	if in.TargetPath == nil {
+		return nil
+	}
+	_, title, err := s.quizzes.QuizOnTarget(ctx, *in.TargetPath, selfID)
+	if err != nil {
+		return err
+	}
+	if title != "" {
+		return fmt.Errorf("%w — %q is already linked to %s. Edit that quiz, or delete it first",
+			repository.ErrQuizTargetTaken, title, *in.TargetPath)
+	}
+	return nil
+}
+
 func (s *QuizService) SetPublished(ctx context.Context, id string, published bool) (*model.Quiz, error) {
 	if published {
 		q, err := s.quizzes.Get(ctx, id, false)