[Django] #34121: Multi Databases documenation example doesn't work

Django

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Oct 26, 2022, 4:58:06 AM10/26/22

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#34121: Multi Databases documenation example doesn't work
-------------------------------------+-------------------------------------
Reporter: MasteroPL | Owner: nobody
Type: Bug | Status: new
Component: Database | Version: 4.1
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
I have been following this documentation:

https://docs.djangoproject.com/en/4.1/topics/db/multi-db/

Here's my Router setup:

{{{
class AuthRouter:
"""
A router to control all database operations on models in the
auth and contenttypes applications.
"""
route_app_labels = ('auth', 'contenttypes')
TARGET_DB = 'users'

def db_for_read(self, model, **hints):
"""
Attempts to read auth and contenttypes models go to auth_db.
"""
if model._meta.app_label in self.route_app_labels:
return self.TARGET_DB
return None

def db_for_write(self, model, **hints):
"""
Attempts to write auth and contenttypes models go to auth_db.
"""
if model._meta.app_label in self.route_app_labels:
return self.TARGET_DB
return None

def allow_relation(self, obj1, obj2, **hints):
"""
Allow relations if a model in the auth or contenttypes apps is
involved.
"""
if (
obj1._meta.app_label in self.route_app_labels or
obj2._meta.app_label in self.route_app_labels
):
return True
return None

def allow_migrate(self, db, app_label, model_name=None, **hints):
"""
Make sure the auth and contenttypes apps only appear in the
'auth_db' database.
"""
if app_label in self.route_app_labels:
return db == self.TARGET_DB
return None
}}}

Here are my settings (kept everything else default):

{{{
DATABASES = {
'default': {
'HOST': 'localhost',
'NAME': 'dev01',
'ENGINE': 'django.db.backends.postgresql',
'USER': 'postgres',
'PASSWORD': 'Admin123!',
'PORT': '5432',
},
'users': {
'HOST': 'localhost',
'NAME': 'dev01',
'ENGINE': 'django.db.backends.postgresql',
'USER': 'postgres',
'PASSWORD': 'Admin123!',
'PORT': '5433',
}
}

DATABASE_ROUTERS = ['multidb.routers.AuthRouter']
}}}

Migrations are applied correctly on "users" database, however when
attempting to apply them on default databse I get an error during {{{
Applying admin.0001_initial...Traceback (most recent call last): }}}

Error states: {{{ django.db.utils.ProgrammingError: relation
"django_content_type" does not exist }}}

Well, logically so - such table is not present in this database. But I
defined the router exactly as the documentation told me to. Therefore
based on what I see in the documentation I labeled it as a bug in this
report.

--
Ticket URL: <https://code.djangoproject.com/ticket/34121>
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Oct 26, 2022, 5:10:35 AM10/26/22

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#34121: Multi Databases documenation example doesn't work
-------------------------------------+-------------------------------------
Reporter: MasteroPL | Owner: nobody

Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by Mariusz Felisiak):

* status: new => closed
* resolution: => invalid

Comment:

> Well, logically so - such table is not present in this database. But I
defined the router exactly as the documentation told me to. Therefore
based on what I see in the documentation I labeled it as a bug in this
report.

All contrib apps must be together in one database, as
[https://docs.djangoproject.com/en/stable/topics/db/multi-db/#no-cross-
database-relations cross-database relations] are not supported. It's
[https://docs.djangoproject.com/en/stable/topics/db/multi-db/#an-example
documented] on the same page:

> ''"your routers must handle the names of all apps in `INSTALLED_APPS`
before you migrate. See
[https://docs.djangoproject.com/en/stable/topics/db/multi-db/#contrib-app-
multiple-databases Behavior of contrib apps] for information about contrib
apps that must be together in one database."''

--
Ticket URL: <https://code.djangoproject.com/ticket/34121#comment:1>

Django

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Oct 26, 2022, 7:19:08 AM10/26/22

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#34121: Multi Databases documenation example doesn't work
-------------------------------------+-------------------------------------

Reporter: Jan Przychodniak | Owner: nobody

Comment (by Jan Przychodniak):

> It's documented on the same page:

>> "your routers must handle the names of all apps in INSTALLED_APPS

before you migrate. See Behavior of contrib apps for information about

contrib apps that must be together in one database."

It is, but I never reached this part, because I got stuck earlier in the
example. When the docs asked me to:

> With this setup installed, and all databases migrated as per
Synchronizing your databases, lets run some Django code:

Which as far as I understand I need to perform migrations first to be able
to run the code. Which migrations I cannot perform based on the example
presented.

I would kindly suggest the example could be updated to mention that you
need to include all the contrib apps in the same database before asking me
to perform the migrations based on the example which never included an app
like "django.contrib.admin" in the router.

Mind you, 'django.contrib.admin' is included in INSTALLED_APPS by default.
I didn't change anything in the settings apart from the piece of code I
sent here.

--
Ticket URL: <https://code.djangoproject.com/ticket/34121#comment:2>

Django

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Oct 26, 2022, 7:35:52 AM10/26/22

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#34121: Multi Databases documenation example doesn't work
-------------------------------------+-------------------------------------
Reporter: Jan Przychodniak | Owner: nobody
Type: Bug | Status: closed
Component: Database layer | Version: 4.1
(models, ORM) |
Severity: Normal | Resolution: invalid
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Comment (by Mariusz Felisiak):

> I would kindly suggest the example could be updated to mention that you
need to include all the contrib apps in the same database before asking me
to perform the migrations based on the example which never included an app
like "django.contrib.admin" in the router.

This section already warns at the very beginning:
> ''"**Example purposes only! ''
>
> ''**This example is intended as a demonstration** of how the router
infrastructure can be used to alter database usage. **It intentionally
ignores some complex issues** in order to demonstrate how routers are
used.''
>
> ''**This example won’t work if any** of the models in myapp contain
relationships to models outside of the other database. Cross-database
relationships introduce referential integrity problems that Django can’t
currently handle.''