package schedule

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"time"
)

var (
	ErrNotFound      = fmt.Errorf("not found")
	ErrAlreadyExists = fmt.Errorf("instances already generated for this period")
)

// ---------------------------------------------------------------------------
// Models
// ---------------------------------------------------------------------------

type ShiftTemplate struct {
	ID           int64          `json:"id"`
	Name         string         `json:"name"`
	DayOfWeek    int            `json:"day_of_week"` // matches time.Weekday: 0=Sun, 6=Sat
	StartTime    string         `json:"start_time"`  // "HH:MM:SS"
	EndTime      string         `json:"end_time"`
	MinCapacity  int            `json:"min_capacity"`
	MaxCapacity  int            `json:"max_capacity"`
	Roles        []TemplateRole `json:"roles"`
	VolunteerIDs []int64        `json:"volunteer_ids"`
	CreatedAt    time.Time      `json:"created_at"`
	UpdatedAt    time.Time      `json:"updated_at"`
}

type TemplateRole struct {
	ID         int64  `json:"id"`
	TemplateID int64  `json:"template_id"`
	RoleName   string `json:"role_name"`
	Count      int    `json:"count"`
}

type ShiftInstance struct {
	ID          int64               `json:"id"`
	TemplateID  *int64              `json:"template_id,omitempty"`
	Name        string              `json:"name"`
	Date        string              `json:"date"` // "YYYY-MM-DD"
	StartTime   string              `json:"start_time"`
	EndTime     string              `json:"end_time"`
	MinCapacity int                 `json:"min_capacity"`
	MaxCapacity int                 `json:"max_capacity"`
	Status      string              `json:"status"` // "draft" or "published"
	Year        int                 `json:"year"`
	Month       int                 `json:"month"`
	Volunteers  []InstanceVolunteer `json:"volunteers"`
	CreatedAt   time.Time           `json:"created_at"`
	UpdatedAt   time.Time           `json:"updated_at"`
}

type InstanceVolunteer struct {
	InstanceID  int64      `json:"instance_id"`
	VolunteerID int64      `json:"volunteer_id"`
	Name        string     `json:"name"`
	Confirmed   bool       `json:"confirmed"`
	ConfirmedAt *time.Time `json:"confirmed_at,omitempty"`
}

// ---------------------------------------------------------------------------
// Input types
// ---------------------------------------------------------------------------

type CreateTemplateInput struct {
	Name         string         `json:"name"`
	DayOfWeek    int            `json:"day_of_week"`
	StartTime    string         `json:"start_time"`
	EndTime      string         `json:"end_time"`
	MinCapacity  int            `json:"min_capacity"`
	MaxCapacity  int            `json:"max_capacity"`
	Roles        []TemplateRole `json:"roles"`
	VolunteerIDs []int64        `json:"volunteer_ids"`
}

type UpdateTemplateInput struct {
	Name         *string        `json:"name"`
	DayOfWeek    *int           `json:"day_of_week"`
	StartTime    *string        `json:"start_time"`
	EndTime      *string        `json:"end_time"`
	MinCapacity  *int           `json:"min_capacity"`
	MaxCapacity  *int           `json:"max_capacity"`
	Roles        []TemplateRole `json:"roles"`
	VolunteerIDs []int64        `json:"volunteer_ids"`
}

type UpdateInstanceInput struct {
	VolunteerIDs *[]int64 `json:"volunteer_ids"`
	MinCapacity  *int     `json:"min_capacity"`
	MaxCapacity  *int     `json:"max_capacity"`
}

// ---------------------------------------------------------------------------
// Store
// ---------------------------------------------------------------------------

type Store struct {
	db *sql.DB
}

func NewStore(db *sql.DB) *Store {
	return &Store{db: db}
}

// ---------------------------------------------------------------------------
// Template operations
// ---------------------------------------------------------------------------

func (s *Store) CreateTemplate(ctx context.Context, in CreateTemplateInput) (*ShiftTemplate, error) {
	tx, err := s.db.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	res, err := tx.ExecContext(ctx,
		`INSERT INTO shift_templates (name, day_of_week, start_time, end_time, min_capacity, max_capacity)
		 VALUES (?, ?, ?, ?, ?, ?)`,
		in.Name, in.DayOfWeek, in.StartTime, in.EndTime, in.MinCapacity, in.MaxCapacity,
	)
	if err != nil {
		return nil, fmt.Errorf("insert template: %w", err)
	}
	id, _ := res.LastInsertId()

	if err := upsertTemplateRoles(ctx, tx, id, in.Roles); err != nil {
		return nil, err
	}
	if err := upsertTemplateVolunteers(ctx, tx, id, in.VolunteerIDs); err != nil {
		return nil, err
	}
	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit: %w", err)
	}
	return s.GetTemplate(ctx, id)
}

func (s *Store) GetTemplate(ctx context.Context, id int64) (*ShiftTemplate, error) {
	t := &ShiftTemplate{}
	var createdAt, updatedAt string
	err := s.db.QueryRowContext(ctx,
		`SELECT id, name, day_of_week, start_time, end_time, min_capacity, max_capacity, created_at, updated_at
		 FROM shift_templates WHERE id = ?`, id,
	).Scan(&t.ID, &t.Name, &t.DayOfWeek, &t.StartTime, &t.EndTime,
		&t.MinCapacity, &t.MaxCapacity, &createdAt, &updatedAt)
	if errors.Is(err, sql.ErrNoRows) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, fmt.Errorf("get template: %w", err)
	}
	t.CreatedAt, _ = time.Parse("2006-01-02 15:04:05", createdAt)
	t.UpdatedAt, _ = time.Parse("2006-01-02 15:04:05", updatedAt)

	roles, err := s.templateRoles(ctx, id)
	if err != nil {
		return nil, err
	}
	t.Roles = roles

	vids, err := s.templateVolunteerIDs(ctx, id)
	if err != nil {
		return nil, err
	}
	t.VolunteerIDs = vids
	return t, nil
}

func (s *Store) ListTemplates(ctx context.Context) ([]ShiftTemplate, error) {
	rows, err := s.db.QueryContext(ctx,
		`SELECT id, name, day_of_week, start_time, end_time, min_capacity, max_capacity, created_at, updated_at
		 FROM shift_templates ORDER BY day_of_week, start_time`)
	if err != nil {
		return nil, fmt.Errorf("list templates: %w", err)
	}
	defer rows.Close()

	var templates []ShiftTemplate
	for rows.Next() {
		var t ShiftTemplate
		var createdAt, updatedAt string
		if err := rows.Scan(&t.ID, &t.Name, &t.DayOfWeek, &t.StartTime, &t.EndTime,
			&t.MinCapacity, &t.MaxCapacity, &createdAt, &updatedAt); err != nil {
			return nil, err
		}
		t.CreatedAt, _ = time.Parse("2006-01-02 15:04:05", createdAt)
		t.UpdatedAt, _ = time.Parse("2006-01-02 15:04:05", updatedAt)
		templates = append(templates, t)
	}
	if err := rows.Err(); err != nil {
		return nil, err
	}

	// Load roles and volunteers for each template
	for i := range templates {
		roles, err := s.templateRoles(ctx, templates[i].ID)
		if err != nil {
			return nil, err
		}
		templates[i].Roles = roles

		vids, err := s.templateVolunteerIDs(ctx, templates[i].ID)
		if err != nil {
			return nil, err
		}
		templates[i].VolunteerIDs = vids
	}
	return templates, nil
}

func (s *Store) UpdateTemplate(ctx context.Context, id int64, in UpdateTemplateInput) (*ShiftTemplate, error) {
	t, err := s.GetTemplate(ctx, id)
	if err != nil {
		return nil, err
	}

	name := t.Name
	dow := t.DayOfWeek
	startTime := t.StartTime
	endTime := t.EndTime
	minCap := t.MinCapacity
	maxCap := t.MaxCapacity

	if in.Name != nil {
		name = *in.Name
	}
	if in.DayOfWeek != nil {
		dow = *in.DayOfWeek
	}
	if in.StartTime != nil {
		startTime = *in.StartTime
	}
	if in.EndTime != nil {
		endTime = *in.EndTime
	}
	if in.MinCapacity != nil {
		minCap = *in.MinCapacity
	}
	if in.MaxCapacity != nil {
		maxCap = *in.MaxCapacity
	}

	tx, err := s.db.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	_, err = tx.ExecContext(ctx,
		`UPDATE shift_templates SET name=?, day_of_week=?, start_time=?, end_time=?,
		 min_capacity=?, max_capacity=?, updated_at=NOW() WHERE id=?`,
		name, dow, startTime, endTime, minCap, maxCap, id,
	)
	if err != nil {
		return nil, fmt.Errorf("update template: %w", err)
	}

	if in.Roles != nil {
		if _, err := tx.ExecContext(ctx, `DELETE FROM shift_template_roles WHERE template_id = ?`, id); err != nil {
			return nil, fmt.Errorf("clear roles: %w", err)
		}
		if err := upsertTemplateRoles(ctx, tx, id, in.Roles); err != nil {
			return nil, err
		}
	}
	if in.VolunteerIDs != nil {
		if _, err := tx.ExecContext(ctx, `DELETE FROM shift_template_volunteers WHERE template_id = ?`, id); err != nil {
			return nil, fmt.Errorf("clear volunteers: %w", err)
		}
		if err := upsertTemplateVolunteers(ctx, tx, id, in.VolunteerIDs); err != nil {
			return nil, err
		}
	}

	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit: %w", err)
	}
	return s.GetTemplate(ctx, id)
}

func (s *Store) DeleteTemplate(ctx context.Context, id int64) error {
	_, err := s.db.ExecContext(ctx, `DELETE FROM shift_templates WHERE id = ?`, id)
	return err
}

// ---------------------------------------------------------------------------
// Instance operations
// ---------------------------------------------------------------------------

// GenerateInstances creates draft shift instances for every template × date in
// the given month. Returns ErrAlreadyExists if instances already exist for
// that month (FR-S02).
func (s *Store) GenerateInstances(ctx context.Context, year, month int) ([]ShiftInstance, error) {
	var count int
	if err := s.db.QueryRowContext(ctx,
		`SELECT COUNT(*) FROM shift_instances WHERE year = ? AND month = ?`, year, month,
	).Scan(&count); err != nil {
		return nil, fmt.Errorf("check existing: %w", err)
	}
	if count > 0 {
		return nil, ErrAlreadyExists
	}

	templates, err := s.ListTemplates(ctx)
	if err != nil {
		return nil, err
	}

	// Find all dates in the month for each template's day of week
	first := time.Date(year, time.Month(month), 1, 0, 0, 0, 0, time.UTC)
	daysInMonth := daysIn(year, month)

	tx, err := s.db.BeginTx(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("begin tx: %w", err)
	}
	defer tx.Rollback()

	var instanceIDs []int64
	for _, tmpl := range templates {
		for d := 0; d < daysInMonth; d++ {
			day := first.AddDate(0, 0, d)
			if int(day.Weekday()) != tmpl.DayOfWeek {
				continue
			}
			res, err := tx.ExecContext(ctx,
				`INSERT INTO shift_instances
				 (template_id, name, date, start_time, end_time, min_capacity, max_capacity, status, year, month)
				 VALUES (?, ?, ?, ?, ?, ?, ?, 'draft', ?, ?)`,
				tmpl.ID, tmpl.Name, day.Format("2006-01-02"),
				tmpl.StartTime, tmpl.EndTime,
				tmpl.MinCapacity, tmpl.MaxCapacity,
				year, month,
			)
			if err != nil {
				return nil, fmt.Errorf("insert instance: %w", err)
			}
			instID, _ := res.LastInsertId()
			instanceIDs = append(instanceIDs, instID)

			// Copy recurring volunteer assignments from template
			for _, vid := range tmpl.VolunteerIDs {
				if _, err := tx.ExecContext(ctx,
					`INSERT IGNORE INTO shift_instance_volunteers (instance_id, volunteer_id) VALUES (?, ?)`,
					instID, vid,
				); err != nil {
					return nil, fmt.Errorf("copy volunteer: %w", err)
				}
			}
		}
	}

	if err := tx.Commit(); err != nil {
		return nil, fmt.Errorf("commit: %w", err)
	}

	return s.ListInstances(ctx, year, month, 0)
}

// ListInstances returns instances for a month. When volunteerID > 0, only
// returns published instances where that volunteer is assigned.
func (s *Store) ListInstances(ctx context.Context, year, month int, volunteerID int64) ([]ShiftInstance, error) {
	query := `SELECT id, template_id, name, date, start_time, end_time,
	           min_capacity, max_capacity, status, year, month, created_at, updated_at
	          FROM shift_instances WHERE year = ? AND month = ?`
	args := []any{year, month}

	if volunteerID > 0 {
		query += ` AND status = 'published'
		           AND id IN (SELECT instance_id FROM shift_instance_volunteers WHERE volunteer_id = ?)`
		args = append(args, volunteerID)
	}
	query += ` ORDER BY date, start_time`

	rows, err := s.db.QueryContext(ctx, query, args...)
	if err != nil {
		return nil, fmt.Errorf("list instances: %w", err)
	}
	defer rows.Close()

	var instances []ShiftInstance
	for rows.Next() {
		inst, err := scanInstance(rows)
		if err != nil {
			return nil, err
		}
		instances = append(instances, *inst)
	}
	if err := rows.Err(); err != nil {
		return nil, err
	}

	for i := range instances {
		vols, err := s.instanceVolunteers(ctx, instances[i].ID)
		if err != nil {
			return nil, err
		}
		instances[i].Volunteers = vols
	}
	return instances, nil
}

func (s *Store) GetInstance(ctx context.Context, id int64) (*ShiftInstance, error) {
	row := s.db.QueryRowContext(ctx,
		`SELECT id, template_id, name, date, start_time, end_time,
		        min_capacity, max_capacity, status, year, month, created_at, updated_at
		 FROM shift_instances WHERE id = ?`, id)
	inst, err := scanInstanceRow(row)
	if errors.Is(err, sql.ErrNoRows) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, fmt.Errorf("get instance: %w", err)
	}
	vols, err := s.instanceVolunteers(ctx, inst.ID)
	if err != nil {
		return nil, err
	}
	inst.Volunteers = vols
	return inst, nil
}

// UpdateInstance edits volunteer assignments and/or capacity on any instance.
// For published instances, volunteer confirmation statuses are reset (FR-S09).
// Returns the previous and new volunteer ID sets so the caller can send notifications.
func (s *Store) UpdateInstance(ctx context.Context, id int64, in UpdateInstanceInput) (inst *ShiftInstance, added []int64, err error) {
	inst, err = s.GetInstance(ctx, id)
	if err != nil {
		return nil, nil, err
	}

	tx, txErr := s.db.BeginTx(ctx, nil)
	if txErr != nil {
		return nil, nil, fmt.Errorf("begin tx: %w", txErr)
	}
	defer tx.Rollback()

	if in.MinCapacity != nil || in.MaxCapacity != nil {
		minCap := inst.MinCapacity
		maxCap := inst.MaxCapacity
		if in.MinCapacity != nil {
			minCap = *in.MinCapacity
		}
		if in.MaxCapacity != nil {
			maxCap = *in.MaxCapacity
		}
		if _, err := tx.ExecContext(ctx,
			`UPDATE shift_instances SET min_capacity=?, max_capacity=?, updated_at=NOW() WHERE id=?`,
			minCap, maxCap, id,
		); err != nil {
			return nil, nil, fmt.Errorf("update capacity: %w", err)
		}
	}

	if in.VolunteerIDs != nil {
		// Determine newly added volunteers (FR-S10)
		existing := make(map[int64]bool)
		for _, v := range inst.Volunteers {
			existing[v.VolunteerID] = true
		}
		for _, vid := range *in.VolunteerIDs {
			if !existing[vid] {
				added = append(added, vid)
			}
		}

		// Replace assignments
		if _, err := tx.ExecContext(ctx,
			`DELETE FROM shift_instance_volunteers WHERE instance_id = ?`, id,
		); err != nil {
			return nil, nil, fmt.Errorf("clear volunteers: %w", err)
		}
		for _, vid := range *in.VolunteerIDs {
			if _, err := tx.ExecContext(ctx,
				`INSERT INTO shift_instance_volunteers (instance_id, volunteer_id) VALUES (?, ?)`,
				id, vid,
			); err != nil {
				return nil, nil, fmt.Errorf("insert volunteer: %w", err)
			}
		}

		// Reset confirmation for published shifts (FR-S09)
		if inst.Status == "published" {
			if _, err := tx.ExecContext(ctx,
				`UPDATE shift_instance_volunteers SET confirmed=0, confirmed_at=NULL WHERE instance_id=?`, id,
			); err != nil {
				return nil, nil, fmt.Errorf("reset confirmations: %w", err)
			}
		}
	}

	if err := tx.Commit(); err != nil {
		return nil, nil, fmt.Errorf("commit: %w", err)
	}

	inst, err = s.GetInstance(ctx, id)
	return inst, added, err
}

// PublishMonth marks all draft instances for the month as published and returns
// a map of volunteerID → []ShiftInstance for notification purposes (FR-S04).
func (s *Store) PublishMonth(ctx context.Context, year, month int) (map[int64][]ShiftInstance, error) {
	if _, err := s.db.ExecContext(ctx,
		`UPDATE shift_instances SET status='published', updated_at=NOW()
		 WHERE year=? AND month=? AND status='draft'`, year, month,
	); err != nil {
		return nil, fmt.Errorf("publish: %w", err)
	}

	instances, err := s.ListInstances(ctx, year, month, 0)
	if err != nil {
		return nil, err
	}

	byVol := make(map[int64][]ShiftInstance)
	for _, inst := range instances {
		for _, v := range inst.Volunteers {
			byVol[v.VolunteerID] = append(byVol[v.VolunteerID], inst)
		}
	}
	return byVol, nil
}

// UnpublishMonth marks all published instances for the month back to draft and
// returns volunteer IDs who had assignments (for notifications FR-S05).
func (s *Store) UnpublishMonth(ctx context.Context, year, month int) ([]int64, error) {
	// Collect affected volunteer IDs before unpublishing
	rows, err := s.db.QueryContext(ctx,
		`SELECT DISTINCT siv.volunteer_id
		 FROM shift_instance_volunteers siv
		 JOIN shift_instances si ON siv.instance_id = si.id
		 WHERE si.year=? AND si.month=? AND si.status='published'`, year, month,
	)
	if err != nil {
		return nil, fmt.Errorf("query volunteers: %w", err)
	}
	defer rows.Close()

	var volunteerIDs []int64
	for rows.Next() {
		var vid int64
		if err := rows.Scan(&vid); err != nil {
			return nil, err
		}
		volunteerIDs = append(volunteerIDs, vid)
	}
	if err := rows.Err(); err != nil {
		return nil, err
	}

	if _, err := s.db.ExecContext(ctx,
		`UPDATE shift_instances SET status='draft', updated_at=NOW()
		 WHERE year=? AND month=? AND status='published'`, year, month,
	); err != nil {
		return nil, fmt.Errorf("unpublish: %w", err)
	}

	return volunteerIDs, nil
}

// ConfirmShift marks a volunteer's attendance confirmation for a shift (FR-S06).
func (s *Store) ConfirmShift(ctx context.Context, instanceID, volunteerID int64) error {
	result, err := s.db.ExecContext(ctx,
		`UPDATE shift_instance_volunteers SET confirmed=1, confirmed_at=NOW()
		 WHERE instance_id=? AND volunteer_id=?`, instanceID, volunteerID,
	)
	if err != nil {
		return fmt.Errorf("confirm shift: %w", err)
	}
	affected, _ := result.RowsAffected()
	if affected == 0 {
		return ErrNotFound
	}
	return nil
}

// ---------------------------------------------------------------------------
// Helpers
// ---------------------------------------------------------------------------

func (s *Store) templateRoles(ctx context.Context, templateID int64) ([]TemplateRole, error) {
	rows, err := s.db.QueryContext(ctx,
		`SELECT id, template_id, role_name, count FROM shift_template_roles WHERE template_id = ?`, templateID)
	if err != nil {
		return nil, fmt.Errorf("get template roles: %w", err)
	}
	defer rows.Close()
	var roles []TemplateRole
	for rows.Next() {
		var r TemplateRole
		if err := rows.Scan(&r.ID, &r.TemplateID, &r.RoleName, &r.Count); err != nil {
			return nil, err
		}
		roles = append(roles, r)
	}
	return roles, rows.Err()
}

func (s *Store) templateVolunteerIDs(ctx context.Context, templateID int64) ([]int64, error) {
	rows, err := s.db.QueryContext(ctx,
		`SELECT volunteer_id FROM shift_template_volunteers WHERE template_id = ?`, templateID)
	if err != nil {
		return nil, fmt.Errorf("get template volunteers: %w", err)
	}
	defer rows.Close()
	var ids []int64
	for rows.Next() {
		var id int64
		if err := rows.Scan(&id); err != nil {
			return nil, err
		}
		ids = append(ids, id)
	}
	return ids, rows.Err()
}

func (s *Store) instanceVolunteers(ctx context.Context, instanceID int64) ([]InstanceVolunteer, error) {
	rows, err := s.db.QueryContext(ctx,
		`SELECT siv.instance_id, siv.volunteer_id, v.name, siv.confirmed, siv.confirmed_at
		 FROM shift_instance_volunteers siv
		 JOIN volunteers v ON v.id = siv.volunteer_id
		 WHERE siv.instance_id = ?
		 ORDER BY v.name`, instanceID,
	)
	if err != nil {
		return nil, fmt.Errorf("get instance volunteers: %w", err)
	}
	defer rows.Close()
	var vols []InstanceVolunteer
	for rows.Next() {
		var iv InstanceVolunteer
		var confirmedAt sql.NullString
		if err := rows.Scan(&iv.InstanceID, &iv.VolunteerID, &iv.Name, &iv.Confirmed, &confirmedAt); err != nil {
			return nil, err
		}
		if confirmedAt.Valid {
			t, _ := time.Parse("2006-01-02 15:04:05", confirmedAt.String)
			iv.ConfirmedAt = &t
		}
		vols = append(vols, iv)
	}
	return vols, rows.Err()
}

func upsertTemplateRoles(ctx context.Context, tx *sql.Tx, templateID int64, roles []TemplateRole) error {
	for _, r := range roles {
		if _, err := tx.ExecContext(ctx,
			`INSERT INTO shift_template_roles (template_id, role_name, count) VALUES (?, ?, ?)`,
			templateID, r.RoleName, r.Count,
		); err != nil {
			return fmt.Errorf("insert role: %w", err)
		}
	}
	return nil
}

func upsertTemplateVolunteers(ctx context.Context, tx *sql.Tx, templateID int64, volunteerIDs []int64) error {
	for _, vid := range volunteerIDs {
		if _, err := tx.ExecContext(ctx,
			`INSERT IGNORE INTO shift_template_volunteers (template_id, volunteer_id) VALUES (?, ?)`,
			templateID, vid,
		); err != nil {
			return fmt.Errorf("insert template volunteer: %w", err)
		}
	}
	return nil
}

type instanceScanner interface {
	Scan(dest ...any) error
}

func scanInstance(r instanceScanner) (*ShiftInstance, error) {
	var inst ShiftInstance
	var templateID sql.NullInt64
	var createdAt, updatedAt string
	if err := r.Scan(&inst.ID, &templateID, &inst.Name, &inst.Date, &inst.StartTime, &inst.EndTime,
		&inst.MinCapacity, &inst.MaxCapacity, &inst.Status, &inst.Year, &inst.Month,
		&createdAt, &updatedAt); err != nil {
		return nil, err
	}
	if templateID.Valid {
		inst.TemplateID = &templateID.Int64
	}
	inst.CreatedAt, _ = time.Parse("2006-01-02 15:04:05", createdAt)
	inst.UpdatedAt, _ = time.Parse("2006-01-02 15:04:05", updatedAt)
	if inst.Volunteers == nil {
		inst.Volunteers = []InstanceVolunteer{}
	}
	return &inst, nil
}

func scanInstanceRow(row *sql.Row) (*ShiftInstance, error) {
	var inst ShiftInstance
	var templateID sql.NullInt64
	var createdAt, updatedAt string
	if err := row.Scan(&inst.ID, &templateID, &inst.Name, &inst.Date, &inst.StartTime, &inst.EndTime,
		&inst.MinCapacity, &inst.MaxCapacity, &inst.Status, &inst.Year, &inst.Month,
		&createdAt, &updatedAt); err != nil {
		return nil, err
	}
	if templateID.Valid {
		inst.TemplateID = &templateID.Int64
	}
	inst.CreatedAt, _ = time.Parse("2006-01-02 15:04:05", createdAt)
	inst.UpdatedAt, _ = time.Parse("2006-01-02 15:04:05", updatedAt)
	if inst.Volunteers == nil {
		inst.Volunteers = []InstanceVolunteer{}
	}
	return &inst, nil
}

func daysIn(year, month int) int {
	return time.Date(year, time.Month(month+1), 0, 0, 0, 0, 0, time.UTC).Day()
}