raising an exception with nested transactions

Author: dantownsend

piccolo/engine/exceptions.py

@@ -0,0 +1,2 @@
+class TransactionError(Exception):
+    pass

piccolo/engine/postgres.py

@@ -12,6 +12,7 @@
 from asyncpg.pool import Pool
 
 from piccolo.engine.base import Batch, Engine
+from piccolo.engine.exceptions import TransactionError
 from piccolo.query.base import Query
 from piccolo.querystring import QueryString
 from piccolo.utils.sync import run_sync
@@ -150,6 +151,11 @@ class Transaction:
 
     def __init__(self, engine: PostgresEngine):
         self.engine = engine
+        if self.engine.transaction_connection.get():
+            raise TransactionError(
+                "A transaction is already active - nested transactions aren't "
+                "currently supported."
+            )
 
     async def __aenter__(self):
         if self.engine.pool:

piccolo/engine/sqlite.py

@@ -9,6 +9,7 @@
 from aiosqlite import Cursor, Connection
 
 from piccolo.engine.base import Batch, Engine
+from piccolo.engine.exceptions import TransactionError
 from piccolo.query.base import Query
 from piccolo.querystring import QueryString
 from piccolo.utils.sync import run_sync
@@ -126,6 +127,11 @@ class Transaction:
 
     def __init__(self, engine: SQLiteEngine):
         self.engine = engine
+        if self.engine.transaction_connection.get():
+            raise TransactionError(
+                "A transaction is already active - nested transactions aren't "
+                "currently supported."
+            )
 
     async def __aenter__(self):
         self.connection = await self.engine.get_connection()

tests/engine/test_nested_transaction.py

@@ -2,10 +2,12 @@
 from unittest import TestCase
 
 from piccolo.columns.column_types import Varchar
+from piccolo.engine.exceptions import TransactionError
 from piccolo.engine.sqlite import SQLiteEngine
 from piccolo.table import Table
 
-from ..base import sqlite_only
+from ..base import sqlite_only, DBTestCase
+from ..example_project.tables import Manager
 
 
 ENGINE_1 = SQLiteEngine(path="engine1.sqlite")
@@ -21,7 +23,7 @@ class Roadie(Table, db=ENGINE_2):
 
 
 @sqlite_only
-class TestNestedTransaction(TestCase):
+class TestDifferentDB(TestCase):
     def setUp(self):
         ENGINE_1.remove_db_file()
         ENGINE_2.remove_db_file()
@@ -53,3 +55,19 @@ async def run_nested(self):
 
     def test_nested(self):
         asyncio.run(self.run_nested())
+
+
+class TestSameDB(DBTestCase):
+    async def run_nested(self):
+        """
+        Nested transactions currently aren't permitted in a connection.
+        """
+        with self.assertRaises(TransactionError):
+            async with Manager._meta.db.transaction():
+                await Manager(name="Bob").save().run()
+
+                async with Manager._meta.db.transaction():
+                    await Manager(name="Dave").save().run()
+
+    def test_nested(self):
+        asyncio.run(self.run_nested())