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[英]Checking if something exists and adding or getting an object from django
[英]Checking that an object exists in DB (Django)
考虑这个 Django 代码:
class User(models.Model):
name = models.CharField(null=True, blank=False, verbose_name=_("Name"), help_text='User Name', max_length=256)
class UsersGroup(models.Model):
name = models.CharField(null=False, blank=False, verbose_name=_("Name"), help_text='Users Group Name', max_length=256)
users = models.ManyToManyField(User)
# ...
with transaction.atomic():
group.users.add(user)
如果在事务开始之前用户被从数据库中删除了怎么办? 它将向group.users
添加一个不存在的用户。 这是一个错误。
在这种情况下该怎么做才能保持数据库的完整性?
如果添加到组中时用户不存在,则查询将在数据库中失败并引发 IntegrityError 消息,如下所示:
IntegrityError: insert or update on table "app1_usersgroup_users" violates foreign key constraint "app1_usersgroup_users_user_id_96d48fc7_fk_polls_user_id"
DETAIL: Key (user_id)=(3) is not present in table "polls_user".
您只需在transaction.atomic
块中添加 get:
with transaction.atomic():
user = User.objects.get(name='stackoverflow')
group.users.add(user)
您也可以使用异常来处理它:
try:
group.users.add(User.objects.get(name='Julian'))
except:
# handle the error - user doesn't exist
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