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具有多对一的关系。 在Session上还有一个Family外键，如下所示：

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的某个特定实例family ，我在session_set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Session的SessionType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>]

似乎select_related方法是为此目的而设计的，但是调用它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()的调用产生了一个包含Session对象而不是SessionType对象的查询集。 如何获取SessionType对象？

Answer 1

我通过过滤所有SessionType对象来解决此问题，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，我有一个暗示，即效率不如仅从family.session_set的Session对象获取相应的session_type ，因此，仍然非常欢迎对为什么select_related()无法按预期工作的解释。

Answer 2

如您所说，您可以使用新的查询集来选择所有SessionType对象。

但是select_related用法不同

根据Django文档，我使用的所有select_related都不会更改queryset对象类型。 但只能通过一个查询从数据库中选择所有相关对象。 例如，请参见以下查询：

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

一次访问数据库，但是在您编写此代码时：

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

对于每次打印，都会发生一个数据库命中，并且发生一个针对基本查询的数据库命中。 这对于少量数据并不重要，但是当数据太大时，如果没有select_related，您的网站就会变慢。 但要注意使用它。 如果使用过多，则操作相反，并且网站运行缓慢。

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