unique quiz constraint

internal/repository/quiz_repo.go

@@ -12,11 +12,14 @@ import (
 )
 
 var (
-	ErrQuizNotFound      = errors.New("quiz not found")
-	ErrQuizNotStarted    = errors.New("start the quiz first")
-	ErrQuizNotCurrent    = errors.New("that isn't the current question")
-	ErrQuizBadOption     = errors.New("that option isn't part of this question")
-	ErrQuizChoiceCount   = errors.New("choose one option, or at least one where several apply")
+	ErrQuizNotFound    = errors.New("quiz not found")
+	ErrQuizNotStarted  = errors.New("start the quiz first")
+	ErrQuizNotCurrent  = errors.New("that isn't the current question")
+	ErrQuizBadOption   = errors.New("that option isn't part of this question")
+	ErrQuizChoiceCount = errors.New("choose one option, or at least one where several apply")
+	// ErrQuizTargetTaken: that chapter, lecture or conversation already has a
+	// quiz (enforced by the quizzes_one_per_target index).
+	ErrQuizTargetTaken   = errors.New("only one quiz is allowed per link")
 	ErrQuizAlreadyAnswer = errors.New("that question is already answered")
 	ErrQuizNotFinished   = errors.New("the quiz isn't finished yet")
 )
@@ -148,6 +151,9 @@ func (r *QuizRepo) Create(ctx context.Context, in QuizInput, createdBy string) (
 		VALUES ($1, $2, $3, $4, $5, $6) RETURNING id`,
 		in.Title, in.Description, in.TargetKind, in.TargetPath, in.TargetTitle, createdBy,
 	).Scan(&id); err != nil {
+		if isUniqueViolation(err, "quizzes_one_per_target") {
+			return "", ErrQuizTargetTaken
+		}
 		return "", fmt.Errorf("create quiz: %w", err)
 	}
 	if err := insertQuestions(ctx, tx, id, in.Questions); err != nil {
@@ -171,6 +177,9 @@ func (r *QuizRepo) Update(ctx context.Context, id string, in QuizInput, replaceQ
 		WHERE id = $1 AND deleted_at IS NULL`,
 		id, in.Title, in.Description, in.TargetKind, in.TargetPath, in.TargetTitle)
 	if err != nil {
+		if isUniqueViolation(err, "quizzes_one_per_target") {
+			return ErrQuizTargetTaken
+		}
 		return fmt.Errorf("update quiz: %w", err)
 	}
 	if tag.RowsAffected() == 0 {
@@ -207,6 +216,23 @@ func insertQuestions(ctx context.Context, tx pgx.Tx, quizID string, qs []Questio
 	return nil
 }
 
+// QuizOnTarget returns the live quiz (published or draft) already attached to
+// a content path, other than exceptID — or "" when the path is free.
+func (r *QuizRepo) QuizOnTarget(ctx context.Context, path, exceptID string) (id, title string, err error) {
+	err = r.pg.Pool.QueryRow(ctx, `
+		SELECT id, title FROM quizzes
+		WHERE target_path = $1 AND deleted_at IS NULL
+		  AND ($2 = '' OR id::text <> $2)
+		LIMIT 1`, path, exceptID).Scan(&id, &title)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return "", "", nil
+	}
+	if err != nil {
+		return "", "", fmt.Errorf("quiz on target: %w", err)
+	}
+	return id, title, nil
+}
+
 func (r *QuizRepo) AttemptCount(ctx context.Context, id string) (int, error) {
 	var n int
 	err := r.pg.Pool.QueryRow(ctx, `SELECT count(*) FROM quiz_attempts WHERE quiz_id = $1`, id).Scan(&n)