filter() values_list()和python列表理解在查询中

I have a quirk(?) with Django queryset filtering:

我有一个关于Django queryset过滤的怪癖(?)

ipdb> MagazineIssue.objects.filter(id__in=l_magazines.values_list('id'))
Out[0]: []

或

ipdb> MagazineIssue.objects.filter(id__in=[l_magazine.id for l_magazine in l_magazines])
Out[0]: [<MagazineIssue: Architecture Australia, Jan 1995 (#1)>]

and

和

ipdb> l_magazines.values_list('id')
Out[0]: [(1,)]
ipdb> [l_magazine.id for l_magazine in l_magazines]
Out[0]: [1]

so, how to use values_list()? (to produce):

那么，如何使用values_list()?(生产):

[1]

or is python list comprehension the 'way to go'?

或者python列表理解是“前进之路”吗?

2 个解决方案

#1

Try l_magazines.values_list('id', flat=True). That returns a list of ids instead of a list of single id tuples.

l_magazines试试。values_list(“id”,平= True)。它返回一个id列表，而不是单个id元组列表。

#2

One thing to note is that there is a difference in the behaviour of values/values_list from a list comprehension:

需要注意的是，值/values_list的行为与列表理解之间存在差异:

values/values_list will yield the actual value stored in the field, that is, just the id (not the whole object)
值/values_list将产生存储在字段中的实际值，即id(不是整个对象)
if the value is a foreign key, and you have the appropriate relations set up in your model, the list comprehension will give you the object referred to by the foreign key.
如果值是外键，并且您在模型中建立了适当的关系，那么列表理解将为您提供外键引用的对象。

Choosing the wrong one will either result in unnecessary database hits, or unnecessary faffing around, depending on what you are trying to do.

选择错误的选项会导致不必要的数据库访问，或者是不必要的浪费，这取决于你想要做什么。

#1