是否可以在 bulk_create() Django 上设置外键？

Question

我需要一次创建许多 EVENT 对象，然后创建许多 ARCHIVED_EVENT 对象，这些对象具有对应事件的外键。

我的代码看起来像这样：

events = []
archivedEvents = []
for _ in range(1000):
    event = Event(name="Test")
    archivedEvent = ArchivedEvent(event_id=event.id)
    archivedEvents.append(archivedEvent)
    events.append(event)

Event.objects.bulk_create(events)
ArchivedEvent.objects.bulk_create(archivedEvents)

不幸的是，这里创建的所有存档事件都有一个指向 EVENT 的 NULL 外键。我知道在将对象保存到数据库之前不会生成对象的主键。但是我在创建存档事件之前保存了这些事件。 我错过了什么吗？ 我应该在批量创建归档事件之前刷新缓存吗？

Answer 1

event.id 不会被创建，除非它通过bulk_create() 创建。可能您可以使用另一个循环来创建ArchivedEvent。像这样：

events = []
archivedEvents = []

for _ in range(1000):
    event = Event(name="Test")
    events.append(event)
events = Event.objects.bulk_create(events)

for event in events:
    archivedEvent = ArchivedEvent(event_id=event.id)
    archivedEvents.append(archivedEvent)
ArchivedEvent.objects.bulk_create(archivedEvents)

根据文档，此解决方案适用于 Postgresql。如果你有其他数据库，那么试试这样：

events = []
archivedEvents = []

for _ in range(1000):
    event = Event(name="Test")
    events.append(event)
Event.objects.bulk_create(events)

events = Event.objects.all().order_by('-id')[:1000][::-1]
for event in events:
    archivedEvent = ArchivedEvent(event_id=event.id)
    archivedEvents.append(archivedEvent)
ArchivedEvent.objects.bulk_create(archivedEvents)

为避免竞争条件，您可以预先生成主键，然后使用它们。例如：

last_event_id = Event.objects.last().id + 1000 # getting last event id and adding thousand to avoid duplicates.

events = [x+last_event_id for x in range(1000)]
archivedEvents = []

for e in events:
    event = Event(name="Test",pk=e)
    events.append(event)
    archivedEvent = ArchivedEvent(event_id=e)
    archivedEvents.append(archivedEvent)

Event.objects.bulk_create(events)
ArchivedEvent.objects.bulk_create(archivedEvents)

Answer 2

这是一个基于@ruddra 的其他数据库解决方案避免使用db 锁的竞争条件的解决方案。希望对您有所帮助。

from django.db import transaction

with transaction.atomic():

   # lock first row to avoid race condition
   # note: first row of Event table must have content,
   #       if not, you need use other tables's not empty row to add lock.
   Event.objects.select_for_update().first()

   # just @ruddra's mysql solution
   events = []
   archivedEvents = []

   for _ in range(1000):
       event = Event(name="Test")
       events.append(event)
   Event.objects.bulk_create(events)

   event_ids = Event.objects.values_list('id', flat=True).order_by('-id')[:1000][::-1]
   for event_id in event_ids:
       archivedEvent = ArchivedEvent(event_id=event_id)
       archivedEvents.append(archivedEvent)
   ArchivedEvent.objects.bulk_create(archivedEvents)