Edit with full solution - Short answer

将@transaction.atomic与select_for_update()一起使用，并手动将会话与save()一起保存.您必须 Select 所有会话，并使用list()进行判断，这样它将锁定每个会话.这是因为存在多用户场景.This isn’t working for SQLite:

@transaction.atomic
def _add_to_cart_atomic(request, product_id, amount):
    p = Product.objects.get(pk=product_id)
    list(Session.objects.select_for_update())  # Force evaluation to lock all the sessions
    if amount > p.number_in_stock - p.number_on_hold:
        return HttpResponse('1')
    if not request.session.get('cart_content', None):
        request.session['cart_content'] = {}
    if request.session['cart_content'].get(product_id, None):
        request.session['cart_content'][product_id] += amount
    else:
        request.session['cart_content'][product_id] = amount
    request.session.save()
    return HttpResponse('0')

def add_to_cart(request):
    if (request.method == "POST"):
        pk = request.POST.get('pk', None)
        amount = int(request.POST.get('amount', 0))
        if pk and amount:
            return _add_to_cart_atomic(request, pk, amount)
    return HttpResponse('', status=404)

Long answer

Using @transaction.atomic alone

使用@transaction.atomic个decorator 似乎解决了一半的问题，即使我不能真正证明这是@transaction.atomic%有效的.这对SQLite和PostgreSQL都有效.

从Wikipedia人起:

原子事务是一系列不可分割且不可减少的数据库操作，它们要么全部发生，要么一个都不发生.原子性保证可以防止发生部分数据库更新，因为它们可能比直接拒绝整个系列造成更大的问题.

@transaction.atomic
def add_to_cart_atomic(request, pk, amount):
    p = Product.objects.get(pk=pk)
    if amount > p.number_in_stock - p.number_on_hold:
        return HttpResponse('1')
    if not request.session.get('cart_content', None):
        request.session['cart_content'] = {}
    if request.session['cart_content'].get(pk, None):
        request.session['cart_content'][pk] += amount
    else:
        request.session['cart_content'][pk] = amount
    request.session.modified = True
    return HttpResponse('0')

def add_to_cart(request):
    if (request.method == "POST"):
        pk = request.POST.get('pk', None)
        amount = int(request.POST.get('amount', 0))
        if pk and amount:
            return add_to_cart_atomic(request, pk, amount)
    return HttpResponse('', status=404)

But it raises another problem.当我重做我的双击javascript测试时，有时，我会收到两个请求的"成功"警告，即使服务器并不真的向购物车添加两次.我认为这是因为删除了SQL查询而没有引发任何错误. 有时，它会按预期工作，第一个请求成功，第二个请求"超出数量".

在当前会话上使用锁定

这差不多是Ilya Khrustalev’s answer个了.如果我们在当前课程中加select_for_update()分.This does not work for SQLite:

@transaction.atomic
def _add_to_cart_atomic(request, product_id, amount):
    p = Product.objects.get(pk=product_id)
    Session.objects.select_for_update().get(session_key = request.session.session_key)
    …

然后这将只锁定当前会话，所以我们的小JavaScript测试将通过.但是像这样的多用户场景呢？:

async def add_one_to_cart(session_key):
    async with aiohttp.ClientSession(cookies={'sessionid': session_key}) as session:
        async with session.post('http://localhost:8000/cart/add-to-cart', data={'pk': 1, 'amount': 1}, cookies={'sessionid': session_key}) as response:
            return await response.text()

async def main():
    result = await asyncio.gather(
        add_one_to_cart('session_key_A'),
        add_one_to_cart('session_key_B'),
        add_one_to_cart('session_key_C')
    )
    print(result)

asyncio.run(main())

结果:

['0', '0', '0']

这意味着来自三个不同会话的每个请求都会通过，即使只剩下一个产品！所以我们有number_on_hold分，超过了2的number_on_stock分！

在所有会话上使用锁定

这是我的"简短回答".