Django中的同一张表中的多重Foriegn键？

Question

我想在Django中创建一个存储数据的模型，其中包含1）总线服务的名称2）源3）目的地4）trip_distance和5）entry_timestamp。

我还想将服务，源和目标的名称保存在单独的数据库表中。所以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) 

这是我想出来的。

现在，当我试图通过壳创建EntryForm，我收到以下错误

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. 

我该怎么设计这个更好？我可以通过管理员手动创建EntryForm对象，然后工作。

在本教程的Django类似的线工作，并没有给这个错误

>>> 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) 

为什么我不能同样创建EntryForm对象？此外，我想要一个表格来保存数据到这个模型，所以我可以命名它？

Answer 1

在评论中，您正在询问如何设置模型。您可以将Source和Destination分组到一个模型中，将其称为Place，因为它们似乎都是指地点。

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) 

只有你Connection模型，你再他们分成source和destination领域。添加related_name使得它更容易阅读，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 

之前，你可以添加Connection S，你需要添加Service和Place对象，连接涉及到。