从Django 2.0迁移到2.2时视图权限及跨库关联报错排查

Let's break down this issue and cover both the root cause and actionable solutions that don't require modifying Django's core code.

What's Causing the Error? #

When upgrading from Django 2.0 to 2.2, you hit this error because Django 2.1 introduced view permissions (the view_<model> permission type). During migrations, Django automatically tries to create these permissions for your models.

The problem arises because:

• Permission objects are linked to ContentType objects (which track your app's models)
• Your custom database router's allow_relation rule was blocking cross-database relations between the Permission (being created in the demo database) and the ContentType (likely stored in your default database)

Solution 1: Adjust Your Custom Database Router #

The simplest fix is updating your router's allow_relation method to explicitly allow relations involving the demo database. This lets Django link the Permission to the ContentType without hitting the router block.

Here's the modified router code:

class AppRouter:
    def allow_relation(self, obj1, obj2, **hints):
        # Allow relations if either object is in the demo database
        if obj1._state.db == "demo" or obj2._state.db == "demo":
            return True
        # Fallback to default rule: only allow same-database relations
        return obj1._state.db == obj2._state.db

After updating the router, re-run your migration command:

python manage.py migrate demo --database demo

Solution 2: Manually Create View Permissions (No Router Changes) #

If you don't want to adjust your router's rules, you can bypass Django's automatic permission generation by creating the view permissions manually in a custom migration. This mimics the core code change you tried, but keeps modifications in your own codebase.

1. Create an empty migration for your demo app:

python manage.py makemigrations demo --empty

2. Open the generated migration file and replace its content with this code (update placeholders to match your models):

from django.db import migrations
from django.contrib.contenttypes.models import ContentType
from django.contrib.auth.models import Permission

def create_view_permissions(apps, schema_editor):
    # Replace 'your_model_name' with the name of your model (lowercase)
    ct = ContentType.objects.get(app_label='demo', model='your_model_name')
    # Create the view permission using content_type_id instead of the object
    Permission.objects.using('demo').get_or_create(
        codename='view_your_model_name',
        name='Can view your model name',
        content_type_id=ct.id
    )

class Migration(migrations.Migration):
    dependencies = [
        # Add the dependency on your app's last migration, e.g.:
        # ('demo', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(create_view_permissions),
    ]

3. Run the migration as usual:

python manage.py migrate demo --database demo

Why Your Core Code Change Worked #

When you modified Django's core django/contrib/auth/management/__init__.py file to use content_type_id=ct.id instead of content_type=ct:

• Originally, passing the full ContentType object triggered Django's database router check (allow_relation), which blocked the cross-database link.
• Using content_type_id directly sets the foreign key's numeric value without passing the object. This skips the router's relation check, so the Permission is saved successfully.

内容的提问来源于stack exchange，提问作者Arpit