Enhance transaction

api/operation.py

@@ -16,6 +16,9 @@ def _remove_transaction(name: str) -> None:
 
 def _remove_operation(name: str, id: int) -> None:
     with conn[name].cursor(row_factory=dict_row) as cur:
+        # can not be >= since there is a tree !
+        cur.execute(f"delete from transaction_operation where id = {id}") # this should not happen
+        cur.execute(f"delete from snapshot_operation where id = {id}") # this may happen
         cur.execute(f"delete from operation where id = {id}")
         return int(cur.fetchone()['id'])
 
@@ -83,7 +86,7 @@ def execute_undo(name: str, discard: bool = False) -> None:
         if bool(tran['strict']): # strict mode disallow undo
             print("Do not allow to undo in strict transaction mode!")
             return
-        elif tran <= curr: # normal mode disallow undo start point
+        elif tran <= curr: # normal mode disallow undo start point, and there is foreign key constraint
             print("Do not allow to undo transaction start point!")
             return
 
@@ -175,7 +178,8 @@ def pick_snapshot(name: str, tag: str) -> None:
                 execute_redo(name)
 
 # transaction is volatile, commit/rollback will destroy transaction.
-# can not undo a committed transaction or redo a rollback transaction.
+# can not redo a rollback transaction.
+# but can undo a committed transaction... inconsistent...
 # it may remove snapshot tag if snapshot in a rollback transaction
 
 def have_transaction(name: str) -> bool: