Django TestCase原子事务失败时数据库未回滚问题求助

问题背景 #

要实现账户间转账的原子事务单元测试，通过transaction.atomic()保证操作要么全部成功要么全部失败。预期验证：当接收方deposit函数失败时，发送方的扣款操作会回滚，余额保持不变。但测试用例失败，显示发送方余额已减少；尝试替换TransactionTestCase无效，仅VS Code逐行调试时测试能通过。

原代码片段 #

import pytest
from cards.models import Card, Account
from unittest.mock import patch

def perform_atomic(txn, sender_account, receiver_account, amount):
    try:
        with transaction.atomic():
            sender_account.balance -= 1000
            sender_account.save()
            receiver_account.deposit(amount)
    except Exception as error:
        txn.status = Transaction.TRANSACTION_STATUS_FAILED
        txn.save()
        logger.error(f"Failed Action. Reason[{error}]", exc_info=True)
        raise error  

# Test case
@patch.object(Account, "deposit")
def test_atomicity_when_deposit_fails(self, deposit_mock):
    deposit_mock.side_effect = ValidationError("deposit failed")
    self.sender_account.balance = 300.0
    self.sender_account.save()

    # Balance before attempting to transfer
    before_sender_balance = self.sender_account.balance
    with pytest.raises(ValidationError, match="deposit failed"):
        perform_atomic(self.transaction, self.sender_account, self.receiver_account, self.amount)

    self.sender_account.refresh_from_db()

    # Balance after attempting to transfer
    after_sender_balance = self.sender_account.balance.amount
    
    # assert that the sender balanc didn't change
    assert before_sender_balance == after_sender_balance

问题原因与修复方案 #

1. 硬编码扣款金额与测试场景冲突 #

原代码中硬编码sender_account.balance -= 1000，但测试里发送方初始余额仅300.0，直接减1000会导致余额负数，可能触发额外未捕获异常，干扰事务回滚逻辑。需改为使用传入的amount参数。

2. 断言时类型不匹配 #

before_sender_balance取的是balance对象，after_sender_balance取的是balance.amount数值，两者类型不同导致断言失败，需统一取值方式。

3. 事务内对象状态与数据库同步问题 #

调试时执行速度慢，对象缓存与数据库状态同步及时；正常运行时事务回滚后，需确保通过refresh_from_db()正确刷新对象状态（原测试已调用，但需配合前两点修复）。

修正后的代码 #

事务执行函数

import pytest
from django.db import transaction
from django.core.exceptions import ValidationError
from cards.models import Account, Transaction
import logging

logger = logging.getLogger(__name__)

def perform_atomic(txn, sender_account, receiver_account, amount):
    try:
        with transaction.atomic():
            # 使用传入的amount替代硬编码值
            sender_account.balance -= amount
            sender_account.save()
            receiver_account.deposit(amount)
        # 转账成功时更新事务状态
        txn.status = Transaction.TRANSACTION_STATUS_SUCCESS
        txn.save()
    except Exception as error:
        txn.status = Transaction.TRANSACTION_STATUS_FAILED
        txn.save()
        logger.error(f"Failed Action. Reason[{error}]", exc_info=True)
        raise error

测试用例

@patch.object(Account, "deposit")
def test_atomicity_when_deposit_fails(self, deposit_mock):
    deposit_mock.side_effect = ValidationError("deposit failed")
    self.amount = 100.0  # 设置合理的转账金额，小于初始余额
    self.sender_account.balance = 300.0
    self.sender_account.save()

    # 统一取数值类型
    before_sender_balance = self.sender_account.balance.amount
    with pytest.raises(ValidationError, match="deposit failed"):
        perform_atomic(self.transaction, self.sender_account, self.receiver_account, self.amount)

    self.sender_account.refresh_from_db()
    after_sender_balance = self.sender_account.balance.amount
    
    assert before_sender_balance == after_sender_balance

内容的提问来源于stack exchange，提问作者Mai Elshiashi