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[英]Django: how to set ForeignKey related_name in Abstract Model class?
[英]ForeignKey field related to abstract model in Django
我有這個型號:
class BaseModel(models.Model):
....
class Meta:
abstract = True
class ModelA(BaseModel):
....
class ModelB(BaseModel):
....
class MyExtModel(models.Model)
myfield = models.ForeignKey(BaseModel)
但這不正確,因為我有像Abstract
BaseModel
。 事實上,當我嘗試makemigration
命令時,我有一個錯誤。
錯誤是:
ERRORS:
myapp.MyExtModel.myfield: (fields.E300) Field defines a relation with model 'BaseModel', which is either not installed, or is abstract.
有沒有辦法使用抽象基礎模型?
我也試過用:
myfield = models.ForeignKey(BaseModel, related_name="%(app_label)s_%(class)s_related")
在Django中無法將外鍵安裝到抽象模型中。 但是,您可以將外鍵安裝到非抽象基類。 唯一的限制是反向外鍵關系將返回基類實例。 您可以使用django-polymorphic來規避此限制。
Django Polymorphic允許您查詢基類對象,但檢索子類實例:
>>> Project.objects.create(topic="Department Party")
>>> ArtProject.objects.create(topic="Painting with Tim", artist="T. Turner")
>>> ResearchProject.objects.create(topic="Swallow Aerodynamics", supervisor="Dr. Winter")
>>> Project.objects.all()
[ <Project: id 1, topic "Department Party">,
<ArtProject: id 2, topic "Painting with Tim", artist "T. Turner">,
<ResearchProject: id 3, topic "Swallow Aerodynamics", supervisor "Dr. Winter"> ]
要使用django polymorphic,您只需要將具有Polymorphic Model的模型聲明為基類:
from django.db import models
from polymorphic.models import PolymorphicModel
class ModelA(PolymorphicModel):
field1 = models.CharField(max_length=10)
class ModelB(ModelA):
field2 = models.CharField(max_length=10)
class ModelC(ModelB):
field3 = models.CharField(max_length=10)
外鍵也將返回子類實例,這是我需要的假設:
# The model holding the relation may be any kind of model, polymorphic or not
class RelatingModel(models.Model):
many2many = models.ManyToManyField('ModelA') # ManyToMany relation to a polymorphic model
>>> o=RelatingModel.objects.create()
>>> o.many2many.add(ModelA.objects.get(id=1))
>>> o.many2many.add(ModelB.objects.get(id=2))
>>> o.many2many.add(ModelC.objects.get(id=3))
>>> o.many2many.all()
[ <ModelA: id 1, field1 (CharField)>,
<ModelB: id 2, field1 (CharField), field2 (CharField)>,
<ModelC: id 3, field1 (CharField), field2 (CharField), field3 (CharField)> ]
考慮到這些查詢的性能稍差 。
當我遇到像我必須使用不同模型的ForeignKeys的情況時,我選擇使用GenericForeignKey
你可以在這里查看官方文檔: Django ContentTypes:Generic Relations
文檔很好地解釋了如何使用它:
from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType
class TaggedItem(models.Model):
tag = models.SlugField()
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')
def __str__(self): # __unicode__ on Python 2
return self.tag
字段content_type存儲通用外鍵指向的模型
字段object_id存儲外鍵的ID,
它不是最好的解決方案,但它可以節省我的一些項目
使用它的例子:
from django.contrib.auth.models import User
guido = User.objects.get(username='Guido')
t = TaggedItem(content_object=guido, tag='bdfl')
t.save()
t.content_object
<User: Guido>
除了GenericForeignKey
的好回答,我不熟悉,有時候(有時候,只要有可能),通過與“基礎”模型使用一對一關系來簡化模型是值得的。
之后使外鍵管理更容易。 如果我記得很清楚,抽象類上的外鍵是不可能的。
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