可能在查询后过滤查询集?詹戈 [英] possible to filter the queryset after querying? django
问题描述
很抱歉，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制作新的查询集.
Sorry if the question sounds weird. I am just wondering if there is possible to make new queryset when I already have a queryset.
例如此处...
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
我的问题是什么...对于not_deleted和is_deleted它们都处于活动状态，它与everyone相同，是否有可能使用everyone，然后以某种方式滤除is_deleted=True还是is_deleted=False?因此，我相信如果可以的话，查询会更快更好.
What my question is...for not_deleted and is_deleted they both have active is true with is the same as everyone is there a possible way to use everyone and then somehow filter out is_deleted=True or is_deleted=False? So then I believe the querying would be faster and better if this is possible right?
然后将所有三个变量everyone，not_deleted和is_deleted用于其他用途.
All three variables everyone, not_deleted and is_deleted will then be used for something else.
希望我的问题已经清楚了.
Hopefully I made my question quiet clear.
谢谢.
推荐答案
是的，您可以重用现有的查询集.
Yes, you can reuse existing querysets.
everyone = User.objects.filter(is_active=True)
active_not_deleted = everyone.filter(is_deleted=False)
active_is_deleted = everyone.filter(is_deleted=True)
这实际上并没有使任何事情变得更快，实际上，因为Django QuerySets是惰性计算的，所以该代码块甚至不会对数据库执行查询.我的意思是，除非您真正需要这些值，否则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.
This is not really making anything faster though, in fact, this code block won't even execute a query against the database because Django QuerySets are lazily evaluated. What I means is that it won't send the query to the database until you actually need the values. Here's an example that will talk to the database.
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
- 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，您不能对列表进行后续过滤，因为它们不是QuerySet.
As an addition to this answer, you could make a single query and then filter in Python if you really wanted. Mind you, you could not do subsequent filtering on the lists because they are not QuerySets.
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，而active_not_deleted和active_is_deleted是User对象的Python列表. everyone查询集仅在第一次list(everyone)调用中被评估一次，然后将结果缓存.
In this last example, everyone is a queryset, and active_not_deleted and active_is_deleted are Python lists of User objects. The everyone queryset will only be evaluated once in the first list(everyone) call, and then the results are cached.
这篇关于可能在查询后过滤查询集?詹戈的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
