ForeignKey field related to abstract model in Django

Question

I have this model:

class BaseModel(models.Model):
    ....

    class Meta:
        abstract = True


class ModelA(BaseModel):
    ....

class ModelB(BaseModel):
    ....


class MyExtModel(models.Model)
    myfield = models.ForeignKey(BaseModel)

But this is not correct because I have BaseModel like Abstract. Infact I have an error when I try makemigration command.

The error is:

ERRORS:
myapp.MyExtModel.myfield: (fields.E300) Field defines a relation with model 'BaseModel', which is either not installed, or is abstract.

Is there a way to use an abstract base model?

I also tried to use:

myfield = models.ForeignKey(BaseModel, related_name="%(app_label)s_%(class)s_related")

Asked By: Safari

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Answer 1

When I faced a situation like that where I have to make ForeignKeys to different models I choose to use GenericForeignKey you can check official docs here: Django ContentTypes: Generic Relations

The docs explain quite well how to use it:

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

The field content_type store the model that the generic foreign key is pointing to
The field object_id store the ID of the foreign key,
The field content_object helps you to access directly to the related object based on the other 2 fields

It is not the best solution but it saves me in some projects

Example of using it:

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>

Answered By: AlvaroAV

Answer 2

Apart from the nice answer with GenericForeignKey, with which I am not quite familiar, sometimes (just sometimes, whenever possible), it pays off to simplify your models with using one-to-one relationships to your ‘base’ model.

Makes foreign keys management easier after that. If I remember well, foreign key on an abstract class is not possible.

Answered By: Wtower

Answer 3

It’s not possible to install Foreign Keys to abstract models in Django.
You can however install Foreign Keys to a non abstract base class. The only limitation is that the reverse Foreign Key relation will return the base class instances.
You can circumvent this limitation by using django-polymorphic.

Django Polymorphic allows you to query the base class objects but retrieves the child class instances:

>>> 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:

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)

Foreign keys will also return the child class instances, which is what you need I assume:

# 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.

Answered By: Sebastian Wozny

ForeignKey field related to abstract model in Django

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