如何在没有count（）的情况下计算queryset中的项数

Question

我想计算我的queryset中返回的项目数。例如

userdesigns = Design.objects.filter (desadder = user.id)

我想用count（）得到返回的对象数，而不使用。

原因是我试图提高性能并减少执行的数据库查询的数量，我注意到使用count（）ping数据库，这是我不想要的。考虑到我已经提取了完整的用户设计，难道不应该有一种方法来计算存储在返回的queryset中的项的数量吗？

Answer 1

len(). A QuerySet is evaluated when you call len() on it. This, as you might expect, returns the length of the result list.

Note: Don't use len() on QuerySets if all you want to do is determine the number of records in the set. It's much more efficient to handle a count at the database level, using SQL's SELECT COUNT(*), and Django provides a count() method for precisely this reason. See count() below.

Source

因此，如果调用len(userdesigns)，而不是userdesigns.count()，django将在单个查询中从表中请求所有相关数据。之后，您可以访问userdesigns变量的所有项，不会进行其他查询。

>>> m = Model1.objects.filter(desadder=1)
>>> len(m)
(0.000) SELECT "app1_model1"."id", "app1_model1"."desadder", "app1_model1"."test" FROM "app1_model1" WHERE "app1_model1"."desadder" = 1 ; args=(1,)
2
>>> m[0]
<Model1: Model1 object>
>>> m = Model1.objects.filter(desadder=1)
>>> len(m)
(0.000) SELECT "app1_model1"."id", "app1_model1"."desadder", "app1_model1"."test" FROM "app1_model1" WHERE "app1_model1"."desadder" = 1 ; args=(1,)
2
>>> m[0]
<Model1: Model1 object>
>>>