sm-quizzes-plugin/classes/service.go

package classes

import (
	"context"
	"errors"
	"math/rand/v2"
	"slices"
	"time"

	"git.golem15.com/golem15/sm-quizzes-plugin/models"
	"git.golem15.com/golem15/summercms/modules/lagoon"
	"gorm.io/gorm"
	"gorm.io/gorm/clause"
)

// Errors the service returns for the API to map onto status codes.
var (
	// ErrNotFound is a missing quiz or an attempt the caller does not own.
	ErrNotFound = errors.New("quizzes: not found")
	// ErrCompleted is a write to a finished attempt.
	ErrCompleted = errors.New("quizzes: attempt already completed")
	// ErrTimeLimit is an answer after the quiz's time limit ran out (plus
	// the grace of a test).
	ErrTimeLimit = errors.New("quizzes: time limit exceeded")
	// ErrAttemptLimit is a test start after the user used every attempt.
	ErrAttemptLimit = errors.New("quizzes: attempt limit reached")
	// ErrAnswerLocked is a changed answer in a one_way test.
	ErrAnswerLocked = errors.New("quizzes: answer locked")
)

// FieldError is an input error on one request field (a 422).
type FieldError struct {
	Field string
	// Key is the golem15.quizzes::lang.api.* phrase of the message.
	Key string
}

func (e *FieldError) Error() string { return "quizzes: invalid " + e.Field + ": " + e.Key }

// Service runs the quiz API on a database. Now defaults to time.Now.
type Service struct {
	DB      *gorm.DB
	Scoring ScoringConfig
	Now     func() time.Time
	// Shuffle reorders questions of a quiz with shuffle_questions; it
	// defaults to math/rand.
	Shuffle func(n int, swap func(i, j int))
	// Grace is how long after a test's deadline an answer is still
	// accepted; it never applies to learning quizzes. See TestGraceFrom.
	Grace time.Duration
}

func (s *Service) now() time.Time {
	if s.Now != nil {
		return s.Now()
	}
	return time.Now()
}

func (s *Service) db(ctx context.Context) *gorm.DB { return s.DB.WithContext(ctx) }

// QuizSummary is one row of the quiz list with the caller's statistics.
type QuizSummary struct {
	Quiz             models.Quiz
	QuestionCount    int
	BestScorePercent *float64
	AttemptsCount    int
	// QuestionTypes counts the questions per type; every models.QuestionTypes
	// key is present.
	QuestionTypes map[string]int
	// AttemptsUsed counts every attempt of the user, open or completed.
	AttemptsUsed int
}

// ListQuizzes returns the published quizzes, optionally only those whose
// context equals quizContext, ordered by title. AttemptsCount counts the
// user's completed attempts, AttemptsUsed every attempt, and
// BestScorePercent is their best result.
func (s *Service) ListQuizzes(ctx context.Context, userID uint, quizContext string) ([]QuizSummary, error) {
	q := s.db(ctx).Where("is_published = ?", true)
	if quizContext != "" {
		q = q.Where("context = ?", quizContext)
	}
	var quizzes []models.Quiz
	if err := q.Order("title ASC, id ASC").Find(&quizzes).Error; err != nil {
		return nil, err
	}
	out := make([]QuizSummary, 0, len(quizzes))
	if len(quizzes) == 0 {
		return out, nil
	}
	ids := make([]uint, len(quizzes))
	for i, quiz := range quizzes {
		ids[i] = quiz.ID
	}
	type countRow struct {
		QuizID uint
		Type   string
		N      int
	}
	var counts []countRow
	if err := s.db(ctx).Model(&models.Question{}).Select("quiz_id, type, COUNT(*) AS n").
		Where("quiz_id IN ?", ids).Group("quiz_id, type").Scan(&counts).Error; err != nil {
		return nil, err
	}
	type statRow struct {
		QuizID uint
		N      int
		Used   int
		Best   *float64
	}
	var stats []statRow
	if err := s.db(ctx).Model(&models.Attempt{}).
		Select("quiz_id, COUNT(*) FILTER (WHERE completed_at IS NOT NULL) AS n, COUNT(*) AS used, "+
			"MAX(score_percent) FILTER (WHERE completed_at IS NOT NULL) AS best").
		Where("quiz_id IN ? AND user_id = ?", ids, userID).
		Group("quiz_id").Scan(&stats).Error; err != nil {
		return nil, err
	}
	typesBy := map[uint]map[string]int{}
	for _, c := range counts {
		if typesBy[c.QuizID] == nil {
			typesBy[c.QuizID] = map[string]int{}
		}
		typesBy[c.QuizID][c.Type] += c.N
	}
	statBy := map[uint]statRow{}
	for _, st := range stats {
		statBy[st.QuizID] = st
	}
	for _, quiz := range quizzes {
		st := statBy[quiz.ID]
		types := make(map[string]int, len(models.QuestionTypes))
		for _, typ := range models.QuestionTypes {
			types[typ] = 0
		}
		total := 0
		for typ, n := range typesBy[quiz.ID] {
			types[typ] += n
			total += n
		}
		out = append(out, QuizSummary{
			Quiz: quiz, QuestionCount: total, BestScorePercent: st.Best, AttemptsCount: st.N,
			QuestionTypes: types, AttemptsUsed: st.Used,
		})
	}
	return out, nil
}

// QuizDetail is a published quiz with its questions and answers.
type QuizDetail struct {
	Quiz       models.Quiz
	Questions  []models.Question
	Categories []string
}

// GetQuiz loads a published quiz by slug with its questions in sort order
// (shuffled when the quiz says so) and each question's answers. A non-empty
// category keeps only that category's questions; a test ignores it, as it
// always covers the whole quiz. Categories lists every category of the quiz.
func (s *Service) GetQuiz(ctx context.Context, slug, category string) (*QuizDetail, error) {
	quiz, err := s.publishedQuiz(ctx, slug)
	if err != nil {
		return nil, err
	}
	if IsTest(quiz) {
		category = ""
	}
	questions, err := s.questions(ctx, quiz.ID, category)
	if err != nil {
		return nil, err
	}
	if quiz.ShuffleQuestions {
		shuffle := s.Shuffle
		if shuffle == nil {
			shuffle = rand.Shuffle
		}
		shuffle(len(questions), func(i, j int) { questions[i], questions[j] = questions[j], questions[i] })
	}
	cats, err := s.categories(ctx, quiz.ID)
	if err != nil {
		return nil, err
	}
	return &QuizDetail{Quiz: *quiz, Questions: questions, Categories: cats}, nil
}

func (s *Service) publishedQuiz(ctx context.Context, slug string) (*models.Quiz, error) {
	var quiz models.Quiz
	err := s.db(ctx).Where("slug = ? AND is_published = ?", slug, true).Take(&quiz).Error
	if errors.Is(err, gorm.ErrRecordNotFound) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, err
	}
	return &quiz, nil
}

func (s *Service) questions(ctx context.Context, quizID uint, category string) ([]models.Question, error) {
	q := s.db(ctx).Where("quiz_id = ?", quizID)
	if category != "" {
		q = q.Where("category = ?", category)
	}
	var questions []models.Question
	err := q.Preload("Answers", func(db *gorm.DB) *gorm.DB { return db.Order("sort_order ASC, id ASC") }).
		Order("sort_order ASC, id ASC").Find(&questions).Error
	return questions, err
}

func (s *Service) categories(ctx context.Context, quizID uint) ([]string, error) {
	cats := []string{}
	err := s.db(ctx).Model(&models.Question{}).
		Where("quiz_id = ? AND category IS NOT NULL AND category <> ''", quizID).
		Distinct("category").Order("category").Pluck("category", &cats).Error
	return cats, err
}

// Started is the attempt POST /quizzes/{slug}/attempts answers with.
type Started struct {
	Attempt *models.Attempt
	Quiz    *models.Quiz
	// Resumed is true when an open test attempt was returned instead of a
	// new one.
	Resumed bool
	// AttemptsUsed counts the user's attempts on the quiz, this one
	// included; it is only counted for tests.
	AttemptsUsed int
	// AnsweredCount is the number of questions a resumed attempt answered.
	AnsweredCount int
}

// StartAttempt starts an attempt at a published quiz for the user, like
// Start without the resume and limit details.
func (s *Service) StartAttempt(ctx context.Context, userID uint, slug, category string) (*models.Attempt, *models.Quiz, error) {
	st, err := s.Start(ctx, userID, slug, category)
	if err != nil {
		return nil, nil, err
	}
	return st.Attempt, st.Quiz, nil
}

// Start starts an attempt at a published quiz for the user.
//
// For a learning quiz a non-empty category limits the attempt to that
// category's questions; a category without questions is a FieldError on
// category. Every call creates a new attempt.
//
// A test always covers the whole quiz, so a category is a FieldError
// (category_not_allowed). Under an advisory lock per (quiz, user) it closes
// the user's open attempts past their deadline and grace at the deadline,
// resumes a remaining open attempt (Resumed) and otherwise refuses with
// ErrAttemptLimit once the user's attempts, open or completed and in any
// mode, reach max_attempts.
func (s *Service) Start(ctx context.Context, userID uint, slug, category string) (*Started, error) {
	quiz, err := s.publishedQuiz(ctx, slug)
	if err != nil {
		return nil, err
	}
	if IsTest(quiz) {
		if category != "" {
			return nil, &FieldError{Field: "category", Key: "category_not_allowed"}
		}
		return s.startTest(ctx, userID, quiz)
	}
	attempt := &models.Attempt{QuizID: quiz.ID, UserID: userID, StartedAt: s.now().UTC()}
	if category != "" {
		var n int64
		if err := s.db(ctx).Model(&models.Question{}).Where("quiz_id = ? AND category = ?", quiz.ID, category).Count(&n).Error; err != nil {
			return nil, err
		}
		if n == 0 {
			return nil, &FieldError{Field: "category", Key: "unknown_category"}
		}
		attempt.Category = &category
	}
	maxPoints, err := s.maxPoints(ctx, s.DB, attempt)
	if err != nil {
		return nil, err
	}
	attempt.MaxPoints = maxPoints
	if err := s.db(ctx).Create(attempt).Error; err != nil {
		return nil, err
	}
	return &Started{Attempt: attempt, Quiz: quiz}, nil
}

func (s *Service) startTest(ctx context.Context, userID uint, quiz *models.Quiz) (*Started, error) {
	var st *Started
	err := lagoon.Transaction(ctx, s.DB, func(ctx context.Context, tx *gorm.DB) error {
		tx = tx.WithContext(ctx)
		if err := tx.Exec(`SELECT pg_advisory_xact_lock(hashtext(?))`, AttemptLockKey(quiz.ID, userID)).Error; err != nil {
			return err
		}
		var open []models.Attempt
		if err := tx.Clauses(clause.Locking{Strength: "UPDATE", Table: clause.Table{Name: clause.CurrentTable}}).
			Where("quiz_id = ? AND user_id = ? AND completed_at IS NULL", quiz.ID, userID).
			Order("started_at ASC, id ASC").Find(&open).Error; err != nil {
			return err
		}
		now := s.now()
		var resume *models.Attempt
		for i := range open {
			a := &open[i]
			a.Quiz = quiz
			if Expired(a, now, s.Grace) {
				if err := s.close(ctx, tx, a, *Deadline(a)); err != nil {
					return err
				}
				continue
			}
			// An open attempt left from the quiz's learning days may cover
			// one category; a test only resumes a whole-quiz attempt.
			if resume == nil && a.Category == nil {
				resume = a
			}
		}
		var used int64
		if err := tx.Model(&models.Attempt{}).Where("quiz_id = ? AND user_id = ?", quiz.ID, userID).Count(&used).Error; err != nil {
			return err
		}
		if resume != nil {
			var answered int64
			if err := tx.Model(&models.AttemptAnswer{}).Where("attempt_id = ?", resume.ID).Count(&answered).Error; err != nil {
				return err
			}
			st = &Started{Attempt: resume, Quiz: quiz, Resumed: true, AttemptsUsed: int(used), AnsweredCount: int(answered)}
			return nil
		}
		if left := AttemptsLeft(quiz, int(used)); left != nil && *left == 0 {
			return ErrAttemptLimit
		}
		attempt := &models.Attempt{QuizID: quiz.ID, UserID: userID, StartedAt: now.UTC()}
		maxPoints, err := s.maxPoints(ctx, tx, attempt)
		if err != nil {
			return err
		}
		attempt.MaxPoints = maxPoints
		if err := tx.Create(attempt).Error; err != nil {
			return err
		}
		st = &Started{Attempt: attempt, Quiz: quiz, AttemptsUsed: int(used) + 1}
		return nil
	})
	if err != nil {
		return nil, err
	}
	return st, nil
}

// AttemptsUsed counts the user's attempts on the quiz, open or completed.
func (s *Service) AttemptsUsed(ctx context.Context, quizID, userID uint) (int, error) {
	var n int64
	err := s.db(ctx).Model(&models.Attempt{}).Where("quiz_id = ? AND user_id = ?", quizID, userID).Count(&n).Error
	return int(n), err
}

func (s *Service) maxPoints(ctx context.Context, db *gorm.DB, a *models.Attempt) (int, error) {
	q := db.WithContext(ctx).Model(&models.Question{}).Where("quiz_id = ?", a.QuizID)
	if a.Category != nil {
		q = q.Where("category = ?", *a.Category)
	}
	var total int
	err := q.Select("COALESCE(SUM(GREATEST(points, 0)), 0)").Scan(&total).Error
	return total, err
}

// ownedAttempt loads the user's attempt with its quiz. Another user's
// attempt is ErrNotFound, never a 403.
func (s *Service) ownedAttempt(ctx context.Context, tx *gorm.DB, userID, attemptID uint, lock bool) (*models.Attempt, error) {
	var attempt models.Attempt
	q := tx.WithContext(ctx).Preload("Quiz").Where("id = ? AND user_id = ?", attemptID, userID)
	if lock {
		q = q.Clauses(clause.Locking{Strength: "UPDATE", Table: clause.Table{Name: clause.CurrentTable}})
	}
	err := q.Take(&attempt).Error
	if errors.Is(err, gorm.ErrRecordNotFound) || (err == nil && attempt.Quiz == nil) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, err
	}
	return &attempt, nil
}

// Deadline is when an attempt's time runs out, or nil without a limit.
func Deadline(a *models.Attempt) *time.Time {
	if a == nil || a.Quiz == nil || a.Quiz.TimeLimitSeconds == nil || *a.Quiz.TimeLimitSeconds <= 0 {
		return nil
	}
	d := a.StartedAt.Add(time.Duration(*a.Quiz.TimeLimitSeconds) * time.Second)
	return &d
}

// AnswerInput is one POST /attempts/{id}/answers body.
type AnswerInput struct {
	QuestionID uint
	AnswerIDs  []uint
	Known      *bool
}

// Feedback is what the user learns after answering one question.
type Feedback struct {
	Question         models.Question
	Result           ScoreResult
	CorrectAnswerIDs []uint
	// Withheld is set for an open test attempt: the solutions stay hidden
	// and the serializer must drop Result, CorrectAnswerIDs and every
	// solution field of Question.
	Withheld bool
}

// Answer records or overwrites the user's answer to one question of an
// open attempt and returns the feedback. It checks, in order: the attempt
// belongs to the user (ErrNotFound), is not finished (ErrCompleted), is
// within the time limit, plus the grace of a test (ErrTimeLimit), the
// question belongs to the attempt (FieldError on question_id), a one_way
// test has no answer to the question yet (ErrAnswerLocked; the attempt row
// is locked, so the check is race-free), and the answer fits the question
// type (FieldError on answer_ids or known). The feedback of an open test
// attempt is Withheld.
func (s *Service) Answer(ctx context.Context, userID, attemptID uint, in AnswerInput) (*Feedback, error) {
	var fb *Feedback
	err := lagoon.Transaction(ctx, s.DB, func(ctx context.Context, tx *gorm.DB) error {
		attempt, err := s.ownedAttempt(ctx, tx, userID, attemptID, true)
		if err != nil {
			return err
		}
		if attempt.CompletedAt != nil {
			return ErrCompleted
		}
		if d := AnswerDeadline(attempt, s.Grace); d != nil && s.now().After(*d) {
			return ErrTimeLimit
		}
		var question models.Question
		q := tx.WithContext(ctx).Preload("Answers", func(db *gorm.DB) *gorm.DB { return db.Order("sort_order ASC, id ASC") }).
			Where("id = ? AND quiz_id = ?", in.QuestionID, attempt.QuizID)
		if attempt.Category != nil {
			q = q.Where("category = ?", *attempt.Category)
		}
		err = q.Take(&question).Error
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return &FieldError{Field: "question_id", Key: "question_not_in_quiz"}
		}
		if err != nil {
			return err
		}
		if OneWay(attempt.Quiz) {
			var given int64
			if err := tx.WithContext(ctx).Model(&models.AttemptAnswer{}).
				Where("attempt_id = ? AND question_id = ?", attempt.ID, question.ID).Count(&given).Error; err != nil {
				return err
			}
			if given > 0 {
				return ErrAnswerLocked
			}
		}

		row := models.AttemptAnswer{AttemptID: attempt.ID, QuestionID: question.ID}
		switch question.Type {
		case models.TypeSelfCheck:
			if in.Known == nil {
				return &FieldError{Field: "known", Key: "known_required"}
			}
			known := *in.Known
			row.SelfCheckKnown = &known
		default:
			if in.AnswerIDs == nil {
				return &FieldError{Field: "answer_ids", Key: "answer_ids_required"}
			}
			ids := uniqueIDs(in.AnswerIDs)
			for _, id := range ids {
				if !slices.ContainsFunc(question.Answers, func(a models.Answer) bool { return a.ID == id }) {
					return &FieldError{Field: "answer_ids", Key: "answer_not_in_question"}
				}
			}
			if question.Type == models.TypeSingle && len(ids) > 1 {
				return &FieldError{Field: "answer_ids", Key: "single_one_answer"}
			}
			row.AnswerIDs = lagoon.Jsonable[[]uint]{Data: ids, Valid: true}
		}

		correct := CorrectAnswerIDs(question)
		res := Score(ScoreInput{
			Type:       question.Type,
			Points:     question.Points,
			CorrectIDs: correct,
			ChosenIDs:  row.AnswerIDs.Data,
			Known:      row.SelfCheckKnown,
		}, s.Scoring)
		row.IsCorrect, row.Points = res.IsCorrect, res.Points
		now := s.now().UTC()
		row.CreatedAt, row.UpdatedAt = now, now
		if err := tx.WithContext(ctx).Clauses(clause.OnConflict{
			Columns:   []clause.Column{{Name: "attempt_id"}, {Name: "question_id"}},
			DoUpdates: clause.AssignmentColumns([]string{"answer_ids", "self_check_known", "is_correct", "points", "updated_at"}),
		}).Create(&row).Error; err != nil {
			return err
		}
		fb = &Feedback{Question: question, Result: res, CorrectAnswerIDs: correct, Withheld: Withheld(attempt)}
		return nil
	})
	if err != nil {
		return nil, err
	}
	return fb, nil
}

// CorrectAnswerIDs lists the IDs of a question's correct answers in sort
// order; it is empty for a self_check question.
func CorrectAnswerIDs(q models.Question) []uint {
	out := []uint{}
	for _, a := range q.Answers {
		if a.IsCorrect {
			out = append(out, a.ID)
		}
	}
	return out
}

// Finish completes an open attempt: it sums the recorded answers, floors
// the total at 0 and stores the score and completion time. It is allowed
// after the time limit, so a late finish still keeps the answers given in
// time.
func (s *Service) Finish(ctx context.Context, userID, attemptID uint) (*AttemptResult, error) {
	err := lagoon.Transaction(ctx, s.DB, func(ctx context.Context, tx *gorm.DB) error {
		attempt, err := s.ownedAttempt(ctx, tx, userID, attemptID, true)
		if err != nil {
			return err
		}
		if attempt.CompletedAt != nil {
			return ErrCompleted
		}
		return s.close(ctx, tx, attempt, s.now())
	})
	if err != nil {
		return nil, err
	}
	return s.Result(ctx, userID, attemptID)
}

// close completes an attempt at the given time: it sums the recorded
// answers, floors the total at 0 and stores the score. Finish closes at the
// current time, an expired test closes at its deadline.
func (s *Service) close(ctx context.Context, tx *gorm.DB, attempt *models.Attempt, at time.Time) error {
	tx = tx.WithContext(ctx)
	var points []float64
	if err := tx.Model(&models.AttemptAnswer{}).Where("attempt_id = ?", attempt.ID).Pluck("points", &points).Error; err != nil {
		return err
	}
	maxPoints, err := s.maxPoints(ctx, tx, attempt)
	if err != nil {
		return err
	}
	score, percent := Totals(points, maxPoints)
	at = at.UTC()
	now := s.now().UTC()
	return tx.Model(&models.Attempt{}).Where("id = ?", attempt.ID).Updates(map[string]any{
		"completed_at":  at,
		"score_points":  score,
		"max_points":    maxPoints,
		"score_percent": percent,
		"updated_at":    now,
	}).Error
}

// AttemptSummary is an attempt with its quiz and answered question count.
type AttemptSummary struct {
	Attempt       models.Attempt
	AnsweredCount int
}

// QuestionResult is one question of an attempt with the user's answer.
type QuestionResult struct {
	Question models.Question
	// Answer is nil when the question was not answered.
	Answer *models.AttemptAnswer
}

// AttemptResult is an attempt with every question it covers.
type AttemptResult struct {
	AttemptSummary
	Questions []QuestionResult
}

// Result loads the user's attempt with every question it covers and the
// answers given. A test attempt past its deadline and grace is closed at
// its deadline first. The questions of a shuffled test come in an order
// seeded by the attempt, the same on every call; other attempts list them
// in sort order.
func (s *Service) Result(ctx context.Context, userID, attemptID uint) (*AttemptResult, error) {
	attempt, err := s.ownedAttempt(ctx, s.DB, userID, attemptID, false)
	if err != nil {
		return nil, err
	}
	if Expired(attempt, s.now(), s.Grace) {
		err := lagoon.Transaction(ctx, s.DB, func(ctx context.Context, tx *gorm.DB) error {
			locked, err := s.ownedAttempt(ctx, tx, userID, attemptID, true)
			if err != nil {
				return err
			}
			if !Expired(locked, s.now(), s.Grace) {
				return nil
			}
			return s.close(ctx, tx, locked, *Deadline(locked))
		})
		if err != nil {
			return nil, err
		}
		if attempt, err = s.ownedAttempt(ctx, s.DB, userID, attemptID, false); err != nil {
			return nil, err
		}
	}
	category := ""
	if attempt.Category != nil {
		category = *attempt.Category
	}
	questions, err := s.questions(ctx, attempt.QuizID, category)
	if err != nil {
		return nil, err
	}
	if IsTest(attempt.Quiz) && attempt.Quiz.ShuffleQuestions {
		rand.New(rand.NewPCG(uint64(attempt.ID), uint64(attempt.QuizID))).
			Shuffle(len(questions), func(i, j int) { questions[i], questions[j] = questions[j], questions[i] })
	}
	var answers []models.AttemptAnswer
	if err := s.db(ctx).Where("attempt_id = ?", attempt.ID).Find(&answers).Error; err != nil {
		return nil, err
	}
	byQuestion := make(map[uint]*models.AttemptAnswer, len(answers))
	for i := range answers {
		byQuestion[answers[i].QuestionID] = &answers[i]
	}
	res := &AttemptResult{AttemptSummary: AttemptSummary{Attempt: *attempt, AnsweredCount: len(answers)}}
	for _, q := range questions {
		res.Questions = append(res.Questions, QuestionResult{Question: q, Answer: byQuestion[q.ID]})
	}
	return res, nil
}

// History lists the user's attempts, newest first, optionally of one quiz
// (by slug, published or not), one page at a time.
func (s *Service) History(ctx context.Context, userID uint, quizSlug string, page, perPage int) ([]AttemptSummary, int64, error) {
	q := s.db(ctx).Model(&models.Attempt{}).Where("golem15_quizzes_attempts.user_id = ?", userID)
	if quizSlug != "" {
		q = q.Joins("JOIN golem15_quizzes_quizzes qz ON qz.id = golem15_quizzes_attempts.quiz_id").Where("qz.slug = ?", quizSlug)
	}
	var total int64
	if err := q.Count(&total).Error; err != nil {
		return nil, 0, err
	}
	var attempts []models.Attempt
	if err := q.Preload("Quiz").Order("golem15_quizzes_attempts.started_at DESC, golem15_quizzes_attempts.id DESC").
		Offset((page - 1) * perPage).Limit(perPage).Find(&attempts).Error; err != nil {
		return nil, 0, err
	}
	out := make([]AttemptSummary, 0, len(attempts))
	if len(attempts) == 0 {
		return out, total, nil
	}
	ids := make([]uint, len(attempts))
	for i, a := range attempts {
		ids[i] = a.ID
	}
	type countRow struct {
		AttemptID uint
		N         int
	}
	var counts []countRow
	if err := s.db(ctx).Model(&models.AttemptAnswer{}).Select("attempt_id, COUNT(*) AS n").
		Where("attempt_id IN ?", ids).Group("attempt_id").Scan(&counts).Error; err != nil {
		return nil, 0, err
	}
	countBy := map[uint]int{}
	for _, c := range counts {
		countBy[c.AttemptID] = c.N
	}
	for _, a := range attempts {
		out = append(out, AttemptSummary{Attempt: a, AnsweredCount: countBy[a.ID]})
	}
	return out, total, nil
}