Django中同一张表中的多个外键？

Question

我想在 Django 中创建一个将数据存储在模型中的表单，该模型包含 1) 巴士服务名称 2) 源 3) 目的地 4)trip_distance 和 5)entry_timestamp。

我还想将服务名称、源名称和目标名称保存在单独的数据库表中。因此，用户可以在填写表单时从下拉菜单中选择这些。

class Service(models.Model):
    service_name = models.CharField(max_length = 30)
    entry_timestamp = models.DateTimeField(auto_now_add = True)

    def __unicode__(self):
        return self.service_name

class Source(models.Model):
    source_name = models.CharField(max_length = 30)
    entry_timestamp = models.DateTimeField(auto_now_add = True)

    def __unicode__(self):
        return self.source_name

class Destination(models.Model):
    destination_name = models.CharField(max_length = 30)
    entry_timestamp = models.DateTimeField(auto_now_add = True)

    def __unicode__(self):
        return self.destination_name

class EntryForm(models.Model):
    service= models.ForeignKey(Service)
    source = models.ForeignKey(Source)
    destination = models.ForeignKey(Destination)
    trip_distance = models.PositiveIntegerField()
    entry_timestamp = models.DateTimeField(auto_now_add = True)

class GlobalOption(models.Model):
    config_option = models.CharField(max_length=30)
    value = models.CharField(max_length = 30)

这是我想出的。

现在，当我尝试通过 shell 创建 EntryForm 时，出现以下错误

q = Service.objects.get(pk=1)
>>> q
<Service: Rajasthan Roadways>
q.entryform_set.create(source = 'Midwest',destination = 'Sahara',trip_distance = 10000)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 752, in create
    return super(RelatedManager, self.db_manager(db)).create(**kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 346, in create
    obj = self.model(**kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/base.py", line 468, in __init__
    setattr(self, field.name, rel_obj)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 635, in __set__
    self.field.rel.to._meta.object_name,
ValueError: Cannot assign "'Midwest'": "EntryForm.source" must be a "Source" instance.

我怎样才能更好地设计这个？我可以通过管理员手动创建 EntryForm 对象，然后它就可以工作了。

在这个 django 教程中，一个类似的行有效并且没有给出这个错误

>>> q = Question.objects.get(pk=1)

# Display any choices from the related object set -- none so far.
>>> q.choice_set.all()
[]

# Create three choices.
>>> q.choice_set.create(choice_text='Not much', votes=0)

为什么我不能类似地创建 EntryForm 对象？我也想要一个表格来将数据保存到这个模型中，我可以命名它吗？

Answer 1

在 cmets 中，您询问如何设置模型。您可以将Source 和Destination 归为一个模型，称之为Place，因为它们似乎都指的是地点。

class Service(models.Model):
    service_name = models.CharField(max_length = 30)
    entry_timestamp = models.DateTimeField(auto_now_add = True)

    def __unicode__(self):
        return self.service_name

class Place(models.Model):
    source_name = models.CharField(max_length = 30)
    entry_timestamp = models.DateTimeField(auto_now_add = True)

    def __unicode__(self):
        return self.source_name

class Connection(models.Model):
    service= models.ForeignKey(Service, related_name='connections')
    source = models.ForeignKey(Place, related_name='departing_connections')
    destination = models.ForeignKey(Place, related_name='arriving_connections')
    trip_distance = models.PositiveIntegerField()
    entry_timestamp = models.DateTimeField(auto_now_add = True)

只有在您的 Connection 模型中，您才能再次将它们分成 source 和 destination 字段。添加related_name 使其更易于阅读，例如

p = Place.objects.get(pk=1)

from_here = p.departing_connections.all()

this_year = from_here.filter(service__entry_timestamp__year=date.today().year

在添加 Connections 之前，您需要添加 Service 和 Place 对象，以便关联的连接。