通用多对多关系

Question

我正在尝试创建一个消息系统，其中消息的发件人和收件人可以是通用实体。这对于发件人来说似乎很好，只有对象可以引用（GenericForeignKey），但我不知道如何为收件人解决这个问题（GenericManyToManyKey ??）

下面是一个简化的例子。 PersonClient 和 CompanyClient 从 Client 继承属性，但有自己的特定细节。最后一行是症结所在。如何让消息接收者成为一组 CompanyClients 和 PersonClients

  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

您可以通过手动创建消息和收件人之间的联结表来使用通用关系来实现这一点：

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)

你会像这样使用上述模型：

>>> 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

如您所见，这是一个更加冗长（复杂？）的解决方案。我可能会保持简单，并在下面使用 Prariedogg 的解决方案。

Answer 2

解决这个问题的绝对最佳方法是使用一个名为 django-gm2m 的库

pip install django-gm2m

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()

我们可以的

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

甜吗？

欲了解更多信息，请访问：

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

Answer 3

您可以通过简化架构以包含单个 Client 表来解决此问题，该表带有一个标志来指示它是什么类型的客户端，而不是使用两个单独的模型。

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)

如果您以这种方式做事，您可能根本不必使用通用外键。作为额外的便利，您还可以为客户端模型编写自定义管理器，例如 Client.corporate.all()、Client.person.all()，以返回仅包含您想要的客户端类型的预过滤查询集。

这也可能不是解决您的问题的最佳方式。我只是把它作为一种潜在的可能性扔在那里。我不知道是否有将两个相似模型组合在一起并使用保存覆盖来确保数据完整性的传统智慧。看起来它可能有潜在的问题......我会让社区在这个方面向我学习。