genmai.go
func (db *DB) defaultFromTag(field *reflect.StructField) (string, error) {
	def := field.Tag.Get(dbDefaultTag)
	if def == "" {
		return "", nil
	}
	switch field.Type.Kind() {
	case reflect.Bool:
		b, err := strconv.ParseBool(def)
		if err != nil {
			return "", err
		}
		return fmt.Sprintf("DEFAULT %v", db.dialect.FormatBool(b)), nil
	}
	return fmt.Sprintf("DEFAULT %v", def), nil
}
func (db *DB) defaultFromTag(field *reflect.StructField) (string, error) {
	def := field.Tag.Get(dbDefaultTag)
	if def == "" {
		return "", nil
	}
	switch field.Type.Kind() {
	case reflect.Bool:
		b, err := strconv.ParseBool(def)
		if err != nil {
			return "", err
		}
		return fmt.Sprintf("DEFAULT %v", db.dialect.FormatBool(b)), nil
	}
	return fmt.Sprintf("DEFAULT %v", def), nil
}
// defaultFromTag returns a "DEFAULT ..." keyword.
// If "default" tag specified to struct field, it use as the default value.
// If it doesn't specify, it returns empty string.