How to use django models with foreign keys in different DBs?

I have 2 models for 2 different databases:
Databases were created manually but it should change nothing.

class LinkModel(models.Model): # in 'urls' database
    id = models.AutoField(primary_key=True)
    host_id = models.IntegerField()
    path = models.CharField(max_length=255)

    class Meta:
        db_table = 'links'
        app_label = 'testapp'

    def __unicode__(self):
        return self.path

class NewsModel(models.Model):  # in default database
    id = models.AutoField(primary_key=True)
    title = models.CharField(max_length=50)
    link = models.ForeignKey(LinkModel)

    class Meta:
        db_table = 'news'
        app_label = 'test'

    def __unicode__(self):
        return self.title

After the following code an error raises

newsItem, created = NewsModel.objects.get_or_create( title="test" )
link = LinkModel.objects.using('urls').get( id=1 )
newsItem.link = link  # error!

 Cannot assign "<LinkModel: />": instance is on database "default", value is on database "urls"

Why can't I use foreign key and a model for different database?

Cross-database limitations

Django doesn't currently provide any support for foreign key or many-to-many relationships spanning multiple databases. If you have used a router to partition models to different databases, any foreign key and many-to-many relationships defined by those models must be internal to a single database.

Django - limitations-of-multiple-databases

Trouble

Same trouble. Bug in ForeignKey() class.

In validate() method.

See ticket

Bug exists in v1.2, v1.3, v1.4rc1

Solution

Try this patch to solve it.

The Problem

*Note: this is an extension of Vitaly Fadeev's answer

Due to a desire to keep referential integrity, Django does not allow for foreign keys which span multiple databases: https://docs.djangoproject.com/en/dev//topics/db/multi-db/#limitations-of-multiple-databases. Although this is desired in 99% of all applications, in some cases it is helpful to be able to create such an association in the ORM even if you cannot ensure referential integrity.

A Solution

I have created a Gist that uses the solution proposed here by Vitaly Fadeev wrapped as a subclass of the Django ForeignKey field. This solution does not require you to modify Django Core files but instead use this field type instead in the cases you need it.

Example Usage

# app1/models.py
from django.db import models

class ClientModel(models.Model)
    name = models.CharField()
    class Meta:
        app_label = 'app1'

# app2/models.py
from django.db import models
from some_location.related import SpanningForeignKey

class WidgetModel(models.Model):
    client = SpanningForeignKey('app1.ClientModel', default=None, null=True,
                                blank=True, verbose_name='Client')

The Gist

The gist is available here: https://gist.github.com/gcko/de1383080e9f8fb7d208

Copied here for ease of access:

from django.core import exceptions
from django.db.models.fields.related import ForeignKey
from django.db.utils import ConnectionHandler, ConnectionRouter

connections = ConnectionHandler()
router = ConnectionRouter()


class SpanningForeignKey(ForeignKey):

    def validate(self, value, model_instance):
        if self.rel.parent_link:
            return
        # Call the grandparent rather than the parent to skip validation
        super(ForeignKey, self).validate(value, model_instance)
        if value is None:
            return

        using = router.db_for_read(self.rel.to, instance=model_instance)
        qs = self.rel.to._default_manager.using(using).filter(
            **{self.rel.field_name: value}
        )
        qs = qs.complex_filter(self.get_limit_choices_to())
        if not qs.exists():
            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={
                    'model': self.rel.to._meta.verbose_name, 'pk': value,
                    'field': self.rel.field_name, 'value': value,
                },  # 'pk' is included for backwards compatibility
            )

As an alternative (a bit hackish though), you could subclass ForeignKey to check for instance existance inside the right db : 

class CrossDbForeignKey(models.ForeignKey):
    def validate(self, value, model_instance):
        if self.rel.parent_link:
            return
        super(models.ForeignKey, self).validate(value, model_instance)
        if value is None:
            return

        # Here is the trick, get db relating to fk, not to root model
        using = router.db_for_read(self.rel.to, instance=model_instance)

        qs = self.rel.to._default_manager.using(using).filter(
                **{self.rel.field_name: value}
             )
        qs = qs.complex_filter(self.rel.limit_choices_to)
        if not qs.exists():
            raise exceptions.ValidationError(self.error_messages['invalid'] % {
                'model': self.rel.to._meta.verbose_name, 'pk': value})

then barely :

class NewsModel(models.Model):  # in default database
    …
    link = models.CrossDbForeignKey(LinkModel)

Note that it corresponds more or less to the patch that Vitaly mentions but that way does not require to patch django source-code.