diff --git a/api/operation.py b/api/operation.py
index 57c2fd9..cedb82c 100644
--- a/api/operation.py
+++ b/api/operation.py
@@ -3,7 +3,7 @@ from .connection import g_conn_dict as conn
 
 def _get_current_transaction(name: str) -> Row | None:
     with conn[name].cursor(row_factory=dict_row) as cur:
-        cur.execute(f"select id from transaction_operation")
+        cur.execute(f"select * from transaction_operation")
         return cur.fetchone()
 
 def _get_current_transaction_id(name: str) -> int:
@@ -20,7 +20,6 @@ def _remove_operation(name: str, id: int) -> None:
         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'])
 
 def _get_parents(name: str, id: int) -> list[int]:
     ids = [id]
@@ -81,14 +80,15 @@ def execute_undo(name: str, discard: bool = False) -> None:
     curr = _get_current_operation(name)
 
     # transaction control
-    tran = _get_current_transaction(name)
-    if int(tran['id']) >= 0:
-        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, and there is foreign key constraint
-            print("Do not allow to undo transaction start point!")
-            return
+    if have_transaction(name):
+        tran = _get_current_transaction(name)
+        if tran != None and int(tran['id']) >= 0:
+            if bool(tran['strict']): # strict mode disallow undo
+                print("Do not allow to undo in strict transaction mode!")
+                return
+            elif int(tran['id']) >= curr: # normal mode disallow undo start point, and there is foreign key constraint
+                print("Do not allow to undo transaction start point!")
+                return
 
     row = _query_undo(name, curr)
     undo = row['undo']
@@ -183,7 +183,9 @@ def pick_snapshot(name: str, tag: str) -> None:
 # it may remove snapshot tag if snapshot in a rollback transaction
 
 def have_transaction(name: str) -> bool:
-    return _get_current_transaction_id(name) >= 0
+    with conn[name].cursor(row_factory=dict_row) as cur:
+        cur.execute(f"select * from transaction_operation")
+        return cur.rowcount > 0
 
 def start_transaction(name: str, strict: bool = False) -> None:
     if have_transaction(name):
@@ -203,11 +205,12 @@ def commit_transaction(name: str) -> None:
     _remove_transaction(name)
 
 def abort_transaction(name: str) -> None:
-    tran = _get_current_transaction_id(name)
-    if tran >= 0:
+    if not have_transaction(name):
         print("No active transaction!")
         return
 
+    tran = _get_current_transaction_id(name)
+
     curr = _get_current_operation(name)
     curr_parents = _get_parents(name, curr)
 
diff --git a/new_demo.py b/new_demo.py
index 5974d8f..d14cc57 100644
--- a/new_demo.py
+++ b/new_demo.py
@@ -1,5 +1,6 @@
 from tjnetwork_new import *
 
+
 def demo_snapshot():
     p = "demo_snapshot"
 
@@ -13,8 +14,8 @@ def demo_snapshot():
     open_project(p)
 
     add_junction(p, 'j-1', 10.0, 20.0, 30.0)
-    add_junction(p, 'j-2', 10.0, 20.0, 30.0) 
-    add_junction(p, 'j-3', 10.0, 20.0, 30.0) 
+    add_junction(p, 'j-2', 10.0, 20.0, 30.0)
+    add_junction(p, 'j-3', 10.0, 20.0, 30.0)
     add_junction(p, 'j-4', 10.0, 20.0, 30.0)
     take_snapshot(p, "1-2-3-4")
 
@@ -35,6 +36,41 @@ def demo_snapshot():
     # delete_project(p)
 
 
+def demo_transaction():
+    p = "demo_transaction"
+
+    if is_project_open(p):
+        close_project(p)
+
+    if have_project(p):
+        delete_project(p)
+
+    create_project(p)
+    open_project(p)
+
+    add_junction(p, 'j-1', 10.0, 20.0, 30.0)
+    take_snapshot(p, "1")
+    add_junction(p, 'j-2', 10.0, 20.0, 30.0)
+    take_snapshot(p, "2")
+
+    start_transaction(p)
+
+    add_junction(p, 'j-3', 10.0, 20.0, 30.0)
+    take_snapshot(p, "3")
+    add_junction(p, 'j-4', 10.0, 20.0, 30.0)
+    take_snapshot(p, "4")
+
+    abort_transaction(p)
+
+    print(have_snapshot(p, "1"))
+    print(have_snapshot(p, "2"))
+    print(have_snapshot(p, "3"))
+    print(have_snapshot(p, "4"))
+
+    close_project(p)
+    # delete_project(p)
+
+
 def demo_1_title():
     p = "demo_1_title"
 
@@ -121,7 +157,8 @@ def demo_2_junctions():
 
 
 if __name__ == "__main__":
-    demo_snapshot()
+    # demo_snapshot()
+    demo_transaction()
     # demo_1_title()
     # demo_2_junctions()
     pass