Django Multi-Database ValueError: "Cannot assign Role object: the current database router prevents this relation"

Question

I'm encountering a ValueError in my Django application when trying to save a User object with a related UserRoleAssociation in the admin interface. The error occurs in a multi-database setup where both the UserRoleAssociation and Role models are intended to use the members database. The error message is:

ValueError: Cannot assign "<Role: Role object (67c43a2e-c4e7-4846-bd31-700bf5d35e82)>": the current database router prevents this relation.

I have a Django project with two databases: default and members. The User, UserRoleAssociation, and Role models are all configured to use the members database via admin classes. The error occurs when saving a User object with an inline UserRoleAssociation in the Django admin.

Models

# models.py
class Role(models.Model):
 id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
 role_name = models.CharField(max_length=255, unique=True)
 class Meta:
 db_table = "roles"
class UserRoleAssociation(models.Model):
 user = models.ForeignKey("User", on_delete=models.CASCADE)
 role = models.ForeignKey("Role", on_delete=models.CASCADE)
 def __str__(self):
 return f"{self.role.role_name}"
 
 class Meta:
 unique_together = ["user", "role"]
 db_table = "user_role_association"
class User(models.Model):
 phone_number = models.CharField(max_length=15)
 is_premium = models.BooleanField(default=False)
 roles = models.ManyToManyField("Role", through=UserRoleAssociation, related_name="users")
 class Meta:
 db_table = "users"
 app_label = "members"
 base_manager_name = 'objects'
 default_manager_name = 'objects'

Admin Configuration I use a custom MultiDBModelAdmin and MultiDBTabularInline to enforce the members database:

# admin.py
class MultiDBModelAdmin(admin.ModelAdmin):
 using = "members"
 def save_model(self, request, obj, form, change):
 obj.save(using=self.using)
 def delete_model(self, request, obj):
 obj._state.db = self.using
 obj.delete(using=self.using)
 
 def delete_queryset(self, request, queryset):
 queryset.using(self.using).delete()
 def get_queryset(self, request):
 return super().get_queryset(request).using(self.using)
 def formfield_for_foreignkey(self, db_field, request, **kwargs):
 return super().formfield_for_foreignkey(db_field, request, using=self.using, **kwargs)
 def formfield_for_manytomany(self, db_field, request, **kwargs):
 return super().formfield_for_manytomany(db_field, request, using=self.using, **kwargs)
 
 def _save_related(self, request, form, formsets, change):
 """Ensure related objects are saved in the correct database."""
 for formset in formsets:
 formset.instance._state.db = self.using # Force the database
 super()._save_related(request, form, formsets, change)
class MultiDBTabularInline(admin.StackedInline):
 using = "members"
 def get_queryset(self, request):
 return super().get_queryset(request).using(self.using)
 def formfield_for_manytomany(self, db_field, request, **kwargs):
= # Tell Django to populate ManyToMany widgets using a query
 # on the 'members' database.
 return super().formfield_for_manytomany(
 db_field, request, using=self.using, **kwargs
 )
class UserRoleAssociationInline(MultiDBTabularInline):
 model = UserRoleAssociation
 extra = 1
@admin.register(User)
class MemberAdmin(MultiDBModelAdmin):
 list_display = ["id"]
 inlines = [UserRoleAssociationInline]

The error occurs when saving a User object with a UserRoleAssociation inline in the admin interface.

Answer 1

Make sure your "DatabaseRouter" allows relation
#routers.py

class MembersRouter:
 def db_for_read(self, model, **hints):
 if model._meta.app_label == "members":
 return "members"
 return None
 def db_for_write(self, model, **hints):
 if model._meta.app_label == "members":
 return "members"
 return None
 def allow_relation(self, obj1, obj2, **hints):
 if obj1._meta.app_label == "members" and obj2._meta.app_label == "members":
 return True
 return None
 def allow_migrate(self, db, app_label, model_name=None, **hints):
 if app_label == "members":
 return db == "members"
 return None

**
Also check you have to add this into your settings.py**
DATABASE_ROUTERS = ['path.to.routers.MembersRouter']

Now, Force correct DB to when saving the inline model "UserRoleSelection"

def _save_related(self, request, form, formsets, change):
 for formset in formsets:
 instances = formset.save(commit=False)
 for obj in instances:
 obj._state.db = self.using
 obj.save(using=self.using)
 formset.save_m2m()