通用多对多关系

Question

我正在尝试创建一个消息系统，在这个系统中，消息的发送者和接收者可以是各种不同的实体。对于发送者来说，这个想法似乎没问题，因为只需要引用一个对象（GenericForeignKey），但我不知道如何处理接收者（是不是要用GenericManyToManyKey？）

下面是一个简化的例子。PersonClient和CompanyClient都继承了Client的属性，但它们有自己特定的细节。最后一行是个难点。怎么才能让消息的接收者既可以是CompanyClients，也可以是PersonClients呢？

  class Client(models.Model):
      city = models.CharField(max_length=16)

      class Meta:
          abstract = True

  class PersonClient(Client):
      first_name = models.CharField(max_length=16)
      last_name = models.CharField(max_length=16)
      gender = models.CharField(max_length=1)

  class CompanyClient(Client):
      name = models.CharField(max_length=32)
      tax_no = PositiveIntegerField()

  class Message(models.Model):
      msg_body = models.CharField(max_length=1024)
      sender = models.ForeignKey(ContentType)
      recipients = models.ManyToManyField(ContentType)

Answer 1

你可以通过简化你的数据库结构来解决这个问题，比如只用一个 Client 表，并加一个标记来说明这个客户是什么类型的，而不是分成两个不同的模型。

from django.db import models
from django.utils.translation import ugettext_lazy as _

class Client(models.Model):
    PERSON, CORPORATION = range(2)
    CLIENT_TYPES = (
                    (PERSON, _('Person')),
                    (CORPORATION, _('Corporation')),
                   )
    type = models.PositiveIntegerField(choices=CLIENT_TYPES, default=PERSON)
    city = models.CharField(max_length=16)
    first_name = models.CharField(max_length=16, blank=True, null=True)
    last_name = models.CharField(max_length=16, blank=True, null=True)
    corporate_name = models.CharField(max_length=16, blank=True, null=True)
    tax_no = models.PositiveIntegerField(blank=True, null=True)

    def save(self, *args, **kwargs):
        """
        Does some validation ensuring that the person specific fields are
        filled in when self.type == self.PERSON, and corporation specific
        fields are filled in when self.type == self.CORPORATION ...

        """
        # conditional save logic goes here
        super(Client, self).save(*args, **kwargs)

这样做的话，你可能就不需要去处理通用外键了。为了方便起见，你还可以为 Client 模型写一些自定义管理器，比如 Client.corporate.all() 和 Client.person.all()，这样可以直接返回你想要的那种客户的筛选结果。

不过，这可能不是解决你问题的最佳方法。我只是提出一个可能的思路。我不知道把两个相似的模型合并在一起，并用保存覆盖来确保数据完整性是否是个常见的做法。这样做可能会有一些问题……我会让社区来告诉我这个问题的答案。

Answer 2

最好的办法是使用一个叫做django-gm2m的库。

pip install django-gm2m

然后如果我们有我们的模型。

>>> from django.db import models
>>>
>>> class Video(models.Model):
>>>       class Meta:
>>>           abstract = True
>>>
>>> class Movie(Video):
>>>     pass
>>>
>>> class Documentary(Video):
>>>     pass

还有一个用户。

>>> from gm2m import GM2MField
>>>
>>> class User(models.Model):
>>>     preferred_videos = GM2MField()

我们可以这样做。

>>> user = User.objects.create()
>>> movie = Movie.objects.create()
>>> documentary = Documentary.objects.create()
>>>
>>> user.preferred_videos.add(movie)
>>> user.preferred_videos.add(documentary)

听起来不错吧？

想了解更多信息可以去这里：

http://django-gm2m.readthedocs.org/en/stable/quick_start.html

Answer 3

你可以通过创建一个通用的关系来实现这个功能，也就是手动建立一个连接表，用来连接消息和接收者：

from django.db import models
from django.contrib.contenttypes import generic
from django.contrib.contenttypes.models import ContentType

class Client(models.Model):
    city = models.CharField(max_length=16)

    # These aren't required, but they'll allow you do cool stuff
    # like "person.sent_messages.all()" to get all messages sent
    # by that person, and "person.received_messages.all()" to
    # get all messages sent to that person.
    # Well...sort of, since "received_messages.all()" will return
    # a queryset of "MessageRecipient" instances.
    sent_messages = generic.GenericRelation('Message',
        content_type_field='sender_content_type',
        object_id_field='sender_id'
    )
    received_messages = generic.GenericRelation('MessageRecipient',
        content_type_field='recipient_content_type',
        object_id_field='recipient_id'
    )

    class Meta:
        abstract = True

class PersonClient(Client):
    first_name = models.CharField(max_length=16)
    last_name = models.CharField(max_length=16)
    gender = models.CharField(max_length=1)

    def __unicode__(self):
        return u'%s %s' % (self.last_name, self.first_name)

class CompanyClient(Client):
    name = models.CharField(max_length=32)
    tax_no = models.PositiveIntegerField()

    def __unicode__(self):
        return self.name

class Message(models.Model):
    sender_content_type = models.ForeignKey(ContentType)
    sender_id = models.PositiveIntegerField()
    sender = generic.GenericForeignKey('sender_content_type', 'sender_id')
    msg_body = models.CharField(max_length=1024)

    def __unicode__(self):
        return u'%s...' % self.msg_body[:25]

class MessageRecipient(models.Model):
    message = models.ForeignKey(Message)
    recipient_content_type = models.ForeignKey(ContentType)
    recipient_id = models.PositiveIntegerField()
    recipient = generic.GenericForeignKey('recipient_content_type', 'recipient_id')

    def __unicode__(self):
        return u'%s sent to %s' % (self.message, self.recipient)

你可以像这样使用上面的模型：

>>> person1 = PersonClient.objects.create(first_name='Person', last_name='One', gender='M')
>>> person2 = PersonClient.objects.create(first_name='Person', last_name='Two', gender='F')
>>> company = CompanyClient.objects.create(name='FastCompany', tax_no='4220')
>>> company_ct = ContentType.objects.get_for_model(CompanyClient)
>>> person_ct = ContentType.objects.get_for_model(person1) # works for instances too.

# now we create a message:

>>> msg = Message.objects.create(sender_content_type=person_ct, sender_id=person1.pk, msg_body='Hey, did any of you move my cheese?')

# and send it to a coupla recipients:

>>> MessageRecipient.objects.create(message=msg, recipient_content_type=person_ct, recipient_id=person2.pk)
>>> MessageRecipient.objects.create(message=msg, recipient_content_type=company_ct, recipient_id=company.pk)
>>> MessageRecipient.objects.count()
2

正如你所看到的，这种方法比较复杂（或者说冗长？）。我可能会选择更简单的方式，采用下面Prariedogg的解决方案。