define default driver name constant (#155)

mysql.go

@@ -22,6 +22,8 @@ import (
 )
 
 const (
+	DefaultDriverName = "mysql"
+
 	AutoRandomTag = "auto_random()" // Treated as an auto_random field for tidb
 )
 
@@ -80,7 +82,7 @@ func New(config Config) gorm.Dialector {
 }
 
 func (dialector Dialector) Name() string {
-	return "mysql"
+	return DefaultDriverName
 }
 
 // NowFunc return now func
@@ -107,7 +109,7 @@ func (dialector Dialector) Apply(config *gorm.Config) error {
 
 func (dialector Dialector) Initialize(db *gorm.DB) (err error) {
 	if dialector.DriverName == "" {
-		dialector.DriverName = "mysql"
+		dialector.DriverName = DefaultDriverName
 	}
 
 	if dialector.DefaultDatetimePrecision == nil {