来自多对多关系的查询集

Question

我正在用Django创建一个小日历应用。我有两个模型类：日历（Calendar）和事件（Event）。一个事件可以出现在多个日历中。为了实现这个功能，我使用了多对多关系（ManyToMany）。

这是我的模型

from django.db import models

class Calendar(models.Model):
    title = models.CharField(max_length = 255)

    def __unicode__(self):
        return self.title

class Event(models.Model):
    title = models.CharField(max_length = 255)
    start_date = models.DateField()
    end_date = models.DateField(blank = True, null = True)
    location = models.CharField(blank = True, max_length = 255)
    description = models.TextField(blank = True)
    important = models.BooleanField(default = False)
    calendar = models.ManyToManyField(Calendar)

我该如何获取一个特定日历中的所有事件呢？

Answer 1

mycalendar = Calendar.objects.get(id=1)

events = mycalendar.event_set.all()

这段代码是从这里拿来并做了修改的： http://docs.djangoproject.com/en/dev/topics/db/queries/#many-to-many-relationships

Answer 2

Django自动提供了一种方法，可以访问在多对多关系中的相关对象：

events = my_calendar.events.all()

你可以查看关于多对多关系的文档。

如果你手头没有日历的实例，而只是有一个ID或者名字，你可以通过一个查询就完成整个操作：

events = Event.objects.filter(calendar__id=my_id)

Answer 3

你可以在一个日历记录的实例上使用 .event_set 属性。用法如下：

# create two calendars
one = models.Calendar.objects.create(title='calendar one')
two = models.Calendar.objects.create(title='calendar two')

# attach event 1 to both calendars
event = models.Event.objects.create(title='event 1', start_date='2011-11-11')
one.event_set.add(event)
two.event_set.add(event)

# attach event 2 to calendar 2
two.event_set.add(models.Event.objects.create(title='event 2', start_date='2011-11-11'))

# get and print all events from calendar one
events_one = models.Calendar.objects.get(title='calendar one').event_set.all()
print [ event.title for event in events_one ] 
# will print: [u'event 1']

# get and print all events from calendar two
events_two = models.Calendar.objects.get(title='calendar two').event_set.all()
print [ event.title for event in events_two ] 
# will print: [u'event 1', u'event 2']

models.Calendar.objects.get(title='two').event_set.all()