Python 如何从Django中的查询集中获取所有相关字段？_Python_Django_Django Queryset

Python 如何从Django中的查询集中获取所有相关字段？

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Python 如何从Django中的查询集中获取所有相关字段？,python,django,django-queryset,Python,Django,Django Queryset,我有两个模型，Session和SessionType，它们具有多对一的关系。在会话上还有一个系列外键，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('Fami

我有两个模型，

Session

和

SessionType

，它们具有多对一的关系。在

会话

上还有一个

系列

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')

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>]>

对于

family

的某个实例

family

，我在

Session\u集中有几个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>]>

该方法似乎就是为了实现这一点，但调用它并不会产生期望的结果：
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>]>

ipdb>family.session\u set.选择相关的（'session\u type'））

如上所述，调用select_related（）
生成了一个包含Session
对象的查询集，而不是SessionType
对象。如何获取SessionType
对象？
我通过过滤所有SessionType
对象解决了这个问题，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')

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>]>

ipdb>SessionType.objects.filter（session\uu family=family）。排序依据（'session\uu session\u number'））

但是，我有一点感觉，这比仅从系列.session\u集合
中的session
对象中获取相应的session\u类型
效率低，因此解释了为什么选择相关（）
未按预期工作，我们仍然非常欢迎。
正如您所说，您可以使用新的查询集选择所有会话类型
对象
但是select\u-related
用法不同
根据Django文档和我使用的所有与select_相关的not change queryset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，请参见此查询：
for item in family.session_set.select_related('session_type').all():
     print item.session_type

单击数据库一次，但在编写此命令时：
for item in family.session_set.all():
     print item.session_type

对于每次打印，都会发生一次数据库命中和一次基本查询的数据库命中。数据太少并不重要，但是当你的数据太大的时候，你的网站就慢了，没有选择相关的。但是要注意使用它。如果你使用它太多的行动是相反的，你的网站去慢
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