python - 查询后可以过滤查询集吗？ Django

Question

对不起，如果这个问题听起来很奇怪。我只是想知道当我已经有一个查询集时是否可以创建新的查询集。

例如这里...

 everyone = User.objects.filter(is_active=True)  # this would of course return all users that's active
 not_deleted = User.objects.filter(is_active=True, is_deleted=False)  # return user that's active and not deleted
 is_deleted = User.objects.filter(is_active=True, is_deleted=True)  # return user that's active and is already deleted

我的问题是...对于 not_deleted 和 is_deleted 他们都处于事件状态是真的，与 everyone 是一样的可能的方式来使用 everyone 然后以某种方式过滤掉 is_deleted=True 或 is_deleted=False？那么我相信如果可能的话，查询会更快更好，对吧？

everyone、not_deleted 和 is_deleted 这三个变量随后将用于其他用途。

希望我把我的问题说得很清楚。

提前致谢。

Answer 1

是的，您可以重复使用现有的查询集。

everyone = User.objects.filter(is_active=True)
active_not_deleted = everyone.filter(is_deleted=False)
active_is_deleted = everyone.filter(is_deleted=True)

虽然这并没有真正使任何事情变得更快，事实上，这个代码块甚至不会对数据库执行查询，因为 Django QuerySet 是延迟评估的。我的意思是，在您真正需要这些值之前，它不会将查询发送到数据库。这是一个与数据库对话的示例。

everyone = User.objects.filter(is_active=True)  # Building SQL...
active_not_deleted = everyone.filter(is_deleted=False)  # Building SQL...
active_is_deleted = everyone.filter(is_deleted=True)  # Building SQL...

# Example of the whole queryset being evaluated
for user in everyone:
    # This will execute the query against the database to return the list of users
    # i.e. "select * from user where is_active is True;"
    print(user)

# Example of using iterator to evaluate one object at a time from the queryset.
for user in active_not_deleted.iterator():
    # This will execute the query for each result, so it doesn't
    # load everything at once and it doesn't cache the results.
    # "select * from user where is_active is True and is_deleted is False limit 1 offset 0;"
    # The offset is incremented on each loop and another query is sent to retrieve the next user in the list.
    print(user)

推荐阅读:

• https://docs.djangoproject.com/en/1.11/topics/db/queries/#querysets-are-lazy
• https://docs.djangoproject.com/en/1.11/ref/models/querysets/#iterator
• https://docs.djangoproject.com/en/1.11/topics/db/queries/#caching-and-querysets

作为此答案的补充，您可以进行单个查询，然后根据需要在 Python 中进行过滤。请注意，您无法对列表进行后续过滤，因为它们不是 QuerySet。

everyone = User.objects.filter(is_active=True)
active_not_deleted = list(filter(lambda user: user.is_deleted is False), list(everyone))
active_is_deleted = list(filter(lambda user: user.is_deleted is True), list(everyone))

在最后一个示例中，everyone 是一个查询集，active_not_deleted 和 active_is_deleted 是用户对象的 Python 列表。 everyone 查询集只会在第一个 list(everyone) 调用中计算一次，然后将结果缓存。