从外键中选择一个字段

问题描述

所以我有一个很简单（简化）的模型

So I have a a very simple (simplified) model

class MyObject(models.Model):
    owning_user = models.ForeignKey(User, null=True)

现在我正在尝试一个我的模板在这些对象的列表中迭代，以确定是否应该显示类似于

Now in one of my templates I'm trying to iterate over a list of these objects to determine whether something should be displayed similar to this

{% for my_object in foo.my_object_set %}
    {% if my_object.owning_user.id == user.id %}
         Show Me!
    {% endif %}

这工作正常，但是我发现是查询

This works fine, but what I am finding is that the query

my_object.owning_user.id

返回所有用户的每个字段，然后再获取在django调试工具栏中进行验证的id，并检查连接查询

returns every field from the owning user before getting the id which is verified both in django debug tool bar, and checking the connection queries

# django-debug-toolbar states this is repeated multiple times
SELECT ••• FROM "auth_user" WHERE "auth_user"."id" = 1

# The following test code also confirms this
from django.db import connection
conn = connection
bearing_type.owning_user.id
print conn.queries[-1]

现在，由于此查询重复超过1000次，每次查询需要2ms，因此只需执行2秒即可所有我关心的是id ...

Now since this query repeats over 1000 times and takes 2ms per query it is taking 2 seconds just to perform this - when all I care about is the id...

无论如何，我只需执行一个查询，只需从 owning_user instea

Is there anyway at all that I can just perform a query to get just the id from the owning_user instead of having to query all the fields?

推荐答案

如果您使用 my_object.owning_user_id 而不是 my_object。 owning_user ，那么Django将使用id而不是查找用户。

If you use my_object.owning_user_id instead of my_object.owning_user, then Django will use the id instead of looking up the user.

在这种情况下，你只需要id，但如果你需要其他用户属性，您可以使用。在视图中，您可以：

In this case, you only need the id, but if you needed other user attributes, you could use select_related. In the view, you would do:

foo.my_object_set.select_related('user')

在模板中，您没有太多的控制权，因为您无法传递参数。

In the template, you don't have as much control, since you can't pass arguments.

{{ foo.my_object_set.select_related }}

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