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Django Custom Model ForeignKey Field for Spanning Databases
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ldsad7 commented Sep 13, 2019

Also searching for an answer as the proposed solution doesn't work for me (django 2.2).

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@ldsad7, as for us, we refused this feature and replaced it with regular microservices.

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Hello. The code has been working well but today I got unexpected error of 'SpanningForeignKey' object has no attribute 'rel'. I looked at the ForeignKey code and it seems like the "rel" attribute has been changed to "remote_field"? And seems to be changed to remote_field.model? Do you need to update the code?
Thanks, Sijian

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joshua2000 commented Apr 27, 2020

**If you are using mysql and django2.2, kindly use the following for the ForeignKey **

**PROBE THANKS TO Jared Scott aka gcko **

from django.core import exceptions
from django.db.models.fields.related import ForeignKey
from django.db.utils import ConnectionHandler, ConnectionRouter
#from django.db.models import ForeignKey
connection = ConnectionHandler()
router = ConnectionRouter()

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

        using = router.db_for_read(self.remote_field.model, instance=model_instance)
        qs = self.remote_field.model._default_manager.using(using).filter(
            **{self.remote_field.field_name: value}

        qs = qs.complex_filter(self.get_limit_choices_to())
        if not qs.exists():
            raise exceptions.ValidationError(
                    'model': self.remote_field.model._meta.verbose_name, 
                    'pk': value,
                    'field': self.remote_field.field_name, 'value': value,
                },  # 'pk' is included for backwards compatibility

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Sarrus1 commented Jul 17, 2020


Thanks you very much for your solution! However, it seems like migrations are not registering the SpanningForeignKey field. Adding them manually does not fix the issue aswell. Do you have any idea what is going wrong?


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@Sarrus1, whats the problem

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is there any solution for manytomanyfield?

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@Sarrus1, cut and paste the error for further help..@diegobill, have not seen one,but keep on looking, the internet is a strange place

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I'm trying to use this class to allow for a lookup of a user in auth_user in a 'default' DB , different from my application DB. ('test2' here).
the table I created looks like:

CREATE TABLE IF NOT EXISTS "test2_m1" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "description" text NOT NULL, "user_id" integer NOT NULL ); -- no foreign key

But when I run these statements, Django generates the following query wth an INNER JOIN to auth_user, and SpanningForeignKey.validate() is not called at all (I added a debug line to test):

from test2.models import *
M1.objects.get(user__username = 'xyz')
db_for_read: test2 ---> test2
ZZZZ SELECT "test2_m1"."id", "test2_m1"."user_id", "test2_m1"."description" FROM "test2_m1" INNER JOIN "auth_user" ON ("test2_m1"."user_id" = "auth_user"."id") WHERE "auth_user"."username" = xyz

The problem here is that the underlying Django code still wants to form a query with an INNER JOIN on the app-specific DB, not the default DB, expecting "auth_user" to be in that DB.

My questions:

  1. How do I get Django to NOT do that INNER JOIN on "auth_user" , especially when I specified no foreign key in the app's DB ? Shouldn't the SpanningForeignKey class prevent this from happening ?
  2. If SpanningForeignKey.validate() won't do that for me, what other method(s) must I override ?

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