Fix AutoMigrate, alterColumn The previous modifications were ignored

[general252]

	// check default value
	if !field.PrimaryKey {
		currentDefaultNotNull := field.HasDefaultValue && (field.DefaultValueInterface != nil || !strings.EqualFold(field.DefaultValue, "NULL"))
		dv, dvNotNull := columnType.DefaultValue()
		if dvNotNull && !currentDefaultNotNull {
			// default value -> null
			alterColumn = true
		} else if !dvNotNull && currentDefaultNotNull {
			// null -> default value
			alterColumn = true
		} else if currentDefaultNotNull || dvNotNull {
			switch field.GORMDataType {
			case schema.Time:
				if !strings.EqualFold(strings.TrimSuffix(dv, "()"), strings.TrimSuffix(field.DefaultValue, "()")) {
					alterColumn = true
				}
			case schema.Bool:
				v1, _ := strconv.ParseBool(dv)
				v2, _ := strconv.ParseBool(field.DefaultValue)
				alterColumn = v1 != v2                                          // ---- 如果alterColumn旧值为true, 应不作修改
			default:
				alterColumn = dv != field.DefaultValue                 // ---- 如果alterColumn旧值为true, 应不作修改
			}
		}
	}

[jinzhu]

Hi @general252

Can you ad some tests?

[general252]

package main

import (
	"fmt"
	"log"

	"gorm.io/driver/mysql"
	"gorm.io/gorm"
)

type WarnMessage struct {
	ID   uint
	Type *string `gorm:"column:type;default:'';size:16"` // update size 16 -> 64
}

func main() {
	var (
		user     = "root"
		password = "123456"
		host     = "127.0.0.1"
		port     = "3306"
		dbname   = "test"
	)

	dsn := fmt.Sprintf("%v:%v@tcp(%v:%v)/%v?parseTime=true", user, password, host, port, dbname)
	e, err := gorm.Open(mysql.New(mysql.Config{
		DriverName: mysql.DefaultDriverName,
		DSN:        dsn,
	}))
	if err != nil {
		return
	}

	err = e.AutoMigrate(new(WarnMessage))
	log.Println(err)
}