Add DB.CommonDB() instead of DB.Tx(), as discussed in the PR thread.

main.go

@@ -28,20 +28,15 @@ func Open(driver, source string) (DB, error) {
 	return db, err
 }
 
-// Return the underlying sql.DB instance.
-//
-// If called inside a transaction, it will panic.
-// Use Tx() instead in this case.
 func (s *DB) DB() *sql.DB {
 	return s.db.(*sql.DB)
 }
 
-// Return the underlying sql.Tx instance.
-//
-// If called outside of a transaction, it will panic.
-// Use DB() instead in this case.
-func (s *DB) Tx() *sql.Tx {
-	return s.db.(*sql.Tx)
+// Return the underlying sql.DB or sql.Tx instance.
+// Use of this method is discouraged. It's mainly intended to allow
+// coexistence with legacy non-GORM code.
+func (s *DB) CommonDB() sqlCommon {
+	return s.db
 }
 
 func (s *DB) Callback() *callback {

main_test.go

@@ -1542,9 +1542,8 @@ func TestTransaction(t *testing.T) {
 		t.Errorf("Should find saved record, but got", err)
 	}
 
-	sql_tx := tx.Tx()  // This shouldn't panic.
-	if sql_tx == nil {
-		t.Errorf("Should return the underlying sql.Tx, but got nil")
+        if sql_tx, ok := tx.CommonDB().(*sql.Tx); !ok || sql_tx == nil {
+                t.Errorf("Should return the underlying sql.Tx")
         }
 
 	tx.Rollback()