[英]Django inheritance and subclass attributes
With a configuration such as this: 使用这样的配置:
class A (models.Model):
common_attribute = models.IntegerField()
class B (A):
subclass_attribute = models.IntegerField()
class C (models.Model)
a = ForeignKey(A)
... if an instance of C contains an instance of B, Django seems to treat this as an instance of A and I can't access casubclass_attribute. ...如果C的实例包含B的实例,则Django似乎将其视为A的实例,因此我无法访问casubclass_attribute。 Is there a way around this? 有没有解决的办法? I don't want to use abstract inheritance because of the difficulties with ForeignKeys - I want C to support any subclass of A. 由于ForeignKeys的困难,我不想使用抽象继承-我希望C支持A的任何子类。
You can circumvent the difficulties with ForeignKey
by using django-polymorphic . 您可以使用django-polymorphic来解决与ForeignKey
的困难。
Django Polymorphic allows you to query the base class objects but retrieves the child class instances: Django Polymorphic允许您查询基类对象,但可以检索子类实例:
>>> 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"> ]
To use django polymorphic you only need to declare your models with Polymorphic Model as base class: 要使用django多态,您只需要使用Polymorphic Model作为基类声明模型:
from django.db import models
from polymorphic 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)
Foreign keys will also return the child class instances, which I think is exactly what you want. 外键还将返回子类实例,我认为这正是您想要的。
# 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)> ]
Take into account that these queries will be slightly less performant . 考虑到这些查询的性能会稍差一些 。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.