Django multiple database foreign keys - django

Question

I am trying to implement multiple database support for my django (version 1.11) app. For that purpose I have included in my settings.py:
DATABASES = {
'default': {
'ENGINE': 'django.db.backends.postgresql_psycopg2',
'NAME': 'labrin_dbase',
'USER': 'labrin_admin',
'PASSWORD': 'ndzwwZHv63STuvAF?C_$L#j#*#epZXaX',
'HOST': 'localhost',
'PORT': '5432',
},
'comment': {
'ENGINE': 'django.db.backends.postgresql_psycopg2',
'NAME': 'labrin_dbase_comments',
'USER': 'labrin_admin_comments',
'PASSWORD': 'adzwaTHv63STuvAF!C_$L#j#*#epZXaY',
'HOST': 'localhost',
'PORT': '5433',
}
}
DATABASE_ROUTERS = [
'labrin_task.comment_router.CommentRouter',
]
And my database router is configured as below:
class CommentRouter(object):
def db_for_read(self, model, **hints):
if model._meta.db_table == 'todo_comment':
return 'comment'
return None
def db_for_write(self, model, **hints):
if model._meta.db_table == 'todo_comment':
return 'comment'
return None
def allow_relation(self, obj1, obj2, **hints):
return True
def allow_migrate(self, db, app_label, model_name=None, **hints):
if model_name == 'comment':
return db == 'comment'
return None
Models in my "todo" app(which is only app in project):
from django.db import models
from django.contrib.auth import get_user_model
UserModel = get_user_model()
class Todo(models.Model):
name = models.CharField(max_length=64)
description = models.TextField()
author = models.ForeignKey(UserModel, on_delete=models.CASCADE)
deadline = models.DateTimeField()
created_at = models.DateTimeField(auto_now_add=True)
class Comment(models.Model):
todo = models.ForeignKey(Todo, on_delete=models.CASCADE)
author = models.ForeignKey(UserModel, on_delete=models.CASCADE)
text = models.CharField(max_length=256)
created_at = models.DateTimeField(auto_now_add=True)
class ShareTodo(models.Model):
todo = models.ForeignKey(Todo, on_delete=models.CASCADE)
with_user = models.ForeignKey(UserModel, on_delete=models.CASCADE)
comment_allowed = models.BooleanField(default=False)
When I remove comment database and DATABASE_ROUTERS from settings.py, my app is working normally. After adding mentioned to settings.py, my app returns an error when I create Comment object. The error says:
Exception inside application: insert or update on table "todo_comment" violates foreign key constraint "todo_comment_author_id_bb272a3e_fk_auth_user_id"
DETAIL: Key (author_id)=(1) is not present in table "auth_user". What am I doing wrong?
Note: I am starting two postgres servers as separate docker containers and after running containers, I run python manage.py migrate and python manage.py migrate --database=comment for making all migrations.

Answer 1

Sorry, but cross-database relations are not possible to recreate in Django. You can find full explanation in Django docs.
Furthermore, you cannot even do cross-database relations in PostgreSQL so even trying to hack it or to achieve it outside of Django won't be possible. Maybe for other database engines it is possible, you can do your own research.