Add DB.Tx() method to provice access to the underlying sql.Tx instance.

main.go

@@ -28,10 +28,22 @@ 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)
+}
+
 func (s *DB) Callback() *callback {
 	s.parent.callback = s.parent.callback.clone()
 	return s.parent.callback

main_test.go

@@ -1542,6 +1542,11 @@ 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")
+	}
+
 	tx.Rollback()
 
 	if err := tx.First(&User{}, "name = ?", "transcation").Error; err == nil {