Django 自定义查询集 class 切片不起作用

Question

我们正在使用自定义对象 (pas1_objects)。一旦我们执行了一些过滤操作，我们就会得到下面的查询集。

all_users_queryset = (
User.pas1_objects.select_related("userlogindetails", "usermeta", "tenant").filter(data_filter)
.exclude(
    userrolepermissions__role_id__in=["PG", "PD"], tenant_id__in=tenant_ids
    )
    .order_by(*sort_array)
)

上面查询的输出是-

<KafkaPushQuerySet [<User: User object (bbc306a1-3a75-4e5e-a9b4-6dce0b31b4b3)>, <User: User object (8830ecc1-944a-43e3-a701-331122a26c2b)>, <User: User object (25255aa5-31f9-45d4-9158-5225fba24cb3)>, <User: User object (4a24f883-210b-43a6-9b57-f51fac624a09)>, <User: User object (b6b044c6-cf9b-46f9-91d9-3a31cd4f6d30)>]>

对该查询集的切片不一致。

(Pdb) all_users_queryset[0:1] # first slice is giving correct output
<KafkaPushQuerySet [<User: User object (bbc306a1-3a75-4e5e-a9b4-6dce0b31b4b3)>]>
(Pdb) all_users_queryset[1:2] # this should return 2nd obj in the query set but it return 1st object
<KafkaPushQuerySet [<User: User object (bbc306a1-3a75-4e5e-a9b4-6dce0b31b4b3)>]>
(Pdb) all_users_queryset[2:3] # this works fine
<KafkaPushQuerySet [<User: User object (25255aa5-31f9-45d4-9158-5225fba24cb3)>]>
(Pdb) all_users_queryset[0:2] # this is returning 1st two objects but in reversed order.
<KafkaPushQuerySet [<User: User object (8830ecc1-944a-43e3-a701-331122a26c2b)>, <User: User object (bbc306a1-3a75-4e5e-a9b4-6dce0b31b4b3)>]>
(Pdb) all_users_queryset[0:3] # this is returning correct objets and in correct order.
<KafkaPushQuerySet [<User: User object (bbc306a1-3a75-4e5e-a9b4-6dce0b31b4b3)>, <User: User object (8830ecc1-944a-43e3-a701-331122a26c2b)>, <User: User object (25255aa5-31f9-45d4-9158-5225fba24cb3)>]>

导致这种不一致的原因是什么？

context on this pas1_object - 这基本上是一个通过继承 models.Manager 创建的自定义对象。并且我们覆盖了 get_queryset，自定义查询集下面 return -

class KafkaPushQuerySet(models.query.QuerySet):
    """
    Overriding queryset function to push to kafka
    """

    def delete(self):
        self.produce_messages_to_kafka(action="delete")
        super(KafkaPushQuerySet, self).delete()

    def update(self, **kwargs):
        super(KafkaPushQuerySet, self).update(**kwargs)
        self.produce_messages_to_kafka(action="update")

谢谢！！！

Answer 1

仅当您对查询集进行切片时才会对其进行评估。换句话说，每次执行切片操作时都会访问数据库（关于这一点，请参见Django documentation）。

那么，问题来了：为什么你的order_by方法每次评估的效果不一样？根据您的 User 数据库 table 和您的变量 sort_array.

的内容，可能有几种（非排他性的）解释

数据库已在查询集的 2 次评估之间更新，
*sort_array对应的元组不长，足够精确，顺序固定且唯一。来自 documentation:

A particular ordering is guaranteed only when ordering by a set of fields that uniquely identify each object in the results. For example, if a name field isn’t unique, ordering by it won’t guarantee objects with the same name always appear in the same order.

一个简单的解决方案是在执行切片之前强制评估查询集，方法是调用 list():

my_list = list(User.pas1_objects.select_related("userlogindetails", "usermeta", "tenant")
    .filter(data_filter)
    .exclude(userrolepermissions__role_id__in=["PG", "PD"], tenant_id__in=tenant_ids)
    .order_by(*sort_array)
)
# And then you can slice your list without any risk of inconsistency
my_list[0:1]

Django 自定义查询集 class 切片不起作用

Django custom queryset class slicing not working

python

django

django-queryset