[Django]-How to use django models with foreign keys in different DBs?

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.

After breaking my head some days, I managed to get my Foreign Key ON THE SAME BANK!

Can be made ​​a change over the FORM to seek a FOREIGN KEY in a different bank!

First, add a RECHARGE of FIELDS, both directly (crack) my form, in function _init_

app.form.py

# -*- coding: utf-8 -*-
from django import forms
import datetime
from app_ti_helpdesk import models as mdp

#classe para formulario de Novo HelpDesk
class FormNewHelpDesk(forms.ModelForm):
    class Meta:
        model = mdp.TblHelpDesk
        fields = (
        "problema_alegado",
        "cod_direcionacao",
        "data_prevista",
        "hora_prevista",
        "atendimento_relacionado_a",
        "status",
        "cod_usuario",
        )

    def __init__(self, *args, **kwargs):
        #-------------------------------------
        #  using remove of kwargs
        #-------------------------------------
        db = kwargs.pop("using", None)

        # CASE use Unique Key`s
        self.Meta.model.db = db

        super(FormNewHelpDesk, self).__init__(*args,**kwargs)

        #-------------------------------------
        #   recreates the fields manually
        from copy import deepcopy
        self.fields = deepcopy( forms.fields_for_model( self.Meta.model, self.Meta.fields, using=db ) )
        #
        #-------------------------------------

        #### follows the standard template customization, if necessary

        self.fields['problema_alegado'].widget.attrs['rows'] = 3
        self.fields['problema_alegado'].widget.attrs['cols'] = 22
        self.fields['problema_alegado'].required = True
        self.fields['problema_alegado'].error_messages={'required': 'Necessário informar o motivo da solicitação de ajuda!'}


        self.fields['data_prevista'].widget.attrs['class'] = 'calendario'
        self.fields['data_prevista'].initial = (datetime.timedelta(4)+datetime.datetime.now().date()).strftime("%Y-%m-%d")

        self.fields['hora_prevista'].widget.attrs['class'] = 'hora'
        self.fields['hora_prevista'].initial =datetime.datetime.now().time().strftime("%H:%M")

        self.fields['status'].initial = '0'                 #aberto
        self.fields['status'].widget.attrs['disabled'] = True

        self.fields['atendimento_relacionado_a'].initial = '07'

        self.fields['cod_direcionacao'].required = True
        self.fields['cod_direcionacao'].label = "Direcionado a"
        self.fields['cod_direcionacao'].initial = '2'
        self.fields['cod_direcionacao'].error_messages={'required': 'Necessário informar para quem é direcionado a ajuda!'}

        self.fields['cod_usuario'].widget = forms.HiddenInput()

calling the Form from the View

app.view.py

form = forms.FormNewHelpDesk(request.POST or None, using=banco)

Now, the change in the source Code DJANGO

Only fields of type ForeignKey, ManyToManyField and OneToOneField can use the ‘using’, so added an IF …

django.forms.models.py

# line - 133: add using=None
def fields_for_model(model, fields=None, exclude=None, widgets=None, formfield_callback=None, using=None):

# line - 159

if formfield_callback is None:
    #----------------------------------------------------
    from django.db.models.fields.related import (ForeignKey, ManyToManyField, OneToOneField)
    if type(f) in (ForeignKey, ManyToManyField, OneToOneField):
        kwargs['using'] = using
    formfield = f.formfield(**kwargs)
    #----------------------------------------------------
elif not callable(formfield_callback):
    raise TypeError('formfield_callback must be a function or callable')
else:
    formfield = formfield_callback(f, **kwargs)

ALTER FOLLOW FILE

django.db.models.base.py

alter

# line 717
qs = model_class._default_manager.filter(**lookup_kwargs)

for

# line 717
qs = model_class._default_manager.using(getattr(self, 'db', None)).filter(**lookup_kwargs)

Ready 😀