make withTx part of SQL

db/db.go

@@ -698,14 +698,8 @@ type txFunc func(tx *sql.Tx) error
 
 // withTx wraps a function in an sql transaction. the transaction is committed if there's no error, or rolled back if there is one.
 // if dbOrTx is an sql.DB, a new transaction is started
-func withTx(dbOrTx interface{}, f txFunc) (err error) {
-	var tx *sql.Tx
-
-	switch t := dbOrTx.(type) {
-	case *sql.Tx:
-		tx = t
-	case *sql.DB:
-		tx, err = t.Begin()
+func (s *SQL) withTx(f txFunc) (err error) {
+	tx, err := s.conn.Begin()
 	if err != nil {
 		return err
 	}
@@ -723,9 +717,6 @@ func withTx(dbOrTx interface{}, f txFunc) (err error) {
 			err = errors.Err(tx.Commit())
 		}
 	}()
-	default:
-		return errors.Err("db or tx required")
-	}
 
 	return f(tx)
 }