如何从 Django 中的查询集中获取所有相关字段？

Question

我有两个模型，Session 和 SessionType，它们具有多对一的关系。在Session 上还有一个Family 外键，像这样：

from django.db import models


class SesssionType(models.Model):
    pass


class Session(models.Model):
    session_type = models.ForeignKey('SessionType')
    family = models.ForeignKey('Family')

对于Family 的某个实例family，我在session_set 中有几个Session 对象：

ipdb> family.session_set.all()
<QuerySet [<Session: Welcome>, <Session: Breastfeeding Preparation - Timothy Anderson>, <Session: First-Time Parents: The Basics of Birth>, <Session: Initial Postpartum Lactation>, <Session: Sleep Techniques for New Babies>, <Session: Breastfeeding Preparation>, <Session: Newborn Care Basics>, <Session: Easing the Transition Back to Work>, <Session: Preparing for Parenting>, <Session: Decoding Baby Cues>, <Session: Postpartum Doula Support>, <Session: First-Time Parents: Birth Prep Q&A>, <Session: Postpartum Lactation Follow-Up>, <Session: Sleep Training for 4 Months & Beyond>, <Session: Mental Wellness in Pregnancy>, <Session: Infant CPR>, <Session: Prenatal Pelvic Physical Therapy>, <Session: Prenatal Massage>]>

我想获得一个包含这些SessionTypes 的SessionTypes 的查询集，类似于以下列表：

ipdb> [session.session_type for session in family.session_set.all()]
[<SessionType: Welcome>, <SessionType: Breastfeeding Preparation>, <SessionType: First-Time Parents: The Basics of Birth>, <SessionType: Initial Postpartum Lactation>, <SessionType: Sleep Techniques for New Babies>, <SessionType: Breastfeeding Preparation>, <SessionType: Newborn Care Basics>, <SessionType: Easing the Transition Back to Work>, <SessionType: Preparing for Parenting>, <SessionType: Decoding Baby Cues>, <SessionType: Postpartum Doula Support>, <SessionType: First-Time Parents: Birth Prep Q&A>, <SessionType: Postpartum Lactation Follow-Up>, <SessionType: Sleep Training for 4 Months & Beyond>, <SessionType: Mental Wellness in Pregnancy>, <SessionType: Infant CPR>, <SessionType: Prenatal Pelvic Physical Therapy>, <SessionType: Prenatal Massage>]

select_related 方法似乎是为此而生的，但调用它并不会产生预期的结果：

ipdb> family.session_set.select_related('session_type')
<QuerySet [<Session: Welcome>, <Session: Breastfeeding Preparation - Timothy Anderson>, <Session: First-Time Parents: The Basics of Birth>, <Session: Initial Postpartum Lactation>, <Session: Sleep Techniques for New Babies>, <Session: Breastfeeding Preparation>, <Session: Newborn Care Basics>, <Session: Easing the Transition Back to Work>, <Session: Preparing for Parenting>, <Session: Decoding Baby Cues>, <Session: Postpartum Doula Support>, <Session: First-Time Parents: Birth Prep Q&A>, <Session: Postpartum Lactation Follow-Up>, <Session: Sleep Training for 4 Months & Beyond>, <Session: Mental Wellness in Pregnancy>, <Session: Infant CPR>, <Session: Prenatal Pelvic Physical Therapy>, <Session: Prenatal Massage>]>

如上所示，对select_related() 的调用产生了一个带有Session 对象的查询集，而不是SessionType 对象。如何获取SessionType 对象？

Answer 1

执行查询以获取一个家庭的所有会话，但只要求返回“session_type”值而不是每个会话的所有数据。

一旦您只返回了 session_types，您就可以使用 distinct() 将该响应归结为唯一值列表。

family.session_set.all().values('session_type').distinct()

Answer 2

正如您所说，您可以使用新查询集来选择所有 SessionType 对象。

但select_related 用法不同

根据 Django 文档和我使用的所有 select_related 不更改查询集对象类型。但只能通过一次查询从数据库中选择所有相关对象。例如看这个查询：

for item in family.session_set.select_related('session_type').all():
     print item.session_type

访问数据库一次，但是当你写这个时：

for item in family.session_set.all():
     print item.session_type

对于每次打印，发生一次数据库命中，并且发生一次基本查询的数据库命中。它对于小数据并不重要，但是当您的数据太大时，如果没有 select_related，您的网站就会变慢。但要注意使用它。如果您使用过多，则相反，您的网站会变慢。

在https://docs.djangoproject.com/en/2.0/ref/models/querysets/#select-related上查看更多信息

Answer 3

我通过过滤所有 SessionType 对象来解决这个问题，如下所示：

ipdb> SessionType.objects.filter(session__family=family).order_by('session__session_number')
<QuerySet [<SessionType: Welcome>, <SessionType: First-Time Parents: The Basics of Birth>, <SessionType: Initial Postpartum Lactation>, <SessionType: Sleep Techniques for New Babies>, <SessionType: Breastfeeding Preparation>, <SessionType: Newborn Care Basics>, <SessionType: Easing the Transition Back to Work>, <SessionType: Preparing for Parenting>, <SessionType: Decoding Baby Cues>, <SessionType: Postpartum Doula Support>, <SessionType: First-Time Parents: Birth Prep Q&A>, <SessionType: Postpartum Lactation Follow-Up>, <SessionType: Sleep Training for 4 Months & Beyond>, <SessionType: Mental Wellness in Pregnancy>, <SessionType: Infant CPR>, <SessionType: Prenatal Pelvic Physical Therapy>, <SessionType: Prenatal Massage>]>

但是，我有一种暗示，这比仅从 family.session_set 中的 Session 对象中获取相应的 session_type 效率低，因此解释为什么 select_related() 无法按预期工作仍然是最重要的欢迎。