package model

import (
	"errors"

	"gorm.io/gorm"
	"gorm.io/gorm/clause"
)

var ErrSessionNotFound = errors.New("会话不存在")

// CreateMessage 在会话行锁保护下分配消息序号，避免并发写入出现重复序号。
func CreateMessage(message *Message) error {
	return DB.Transaction(func(tx *gorm.DB) error {
		var session Session
		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).First(&session, message.SessionID).Error; err != nil {
			if errors.Is(err, gorm.ErrRecordNotFound) {
				return ErrSessionNotFound
			}
			return err
		}

		var maxSeq int
		if err := tx.Model(&Message{}).
			Where("session_id = ?", message.SessionID).
			Select("COALESCE(MAX(seq), 0)").
			Scan(&maxSeq).Error; err != nil {
			return err
		}

		message.Seq = maxSeq + 1
		return tx.Create(message).Error
	})
}