Using PostgresDsn for database settings parsing broken on Pydantic 2.10.0 onward

[attevaltojarvi]

Hi,

after upgrading my Pydantic version to 2.10.x, I noticed that my Pydjantic settings for my project prevent it from starting up. This is my settings definition:

class DatabaseSettings(pydjantic.BaseDBConfig):
    default: t.Annotated[
        pydantic.PostgresDsn, pydantic.Field(
            validation_alias='DATABASE_URL',
            json_schema_extra={'test_options': {'MIGRATE': False}}
        )
    ]
    
class DjangoSettings(BaseSettings):
    DATABASES: DatabaseSettings = DatabaseSettings()

The crash happens in django.db.utils, in the configure_settings method, where the line conn.setdefault("ATOMIC_REQUESTS", False) raises AttributeError: 'PostgresDsn' object has no attribute 'setdefault'.

This seems to be due to a Pydantic internal change (I'm suspecting this PR: pydantic/pydantic#10662), causing the if not isinstance(value, (str, MultiHostUrl)): check to be hit in the field validator method of the BaseDBConfig class, and the value, the PostgresDsn instance, being returned as-is. Apparently there's some inheritance change.

This can be prevented by importing the private base class from which PostgresDsn currently inherits from and adding it in the tuple:

from pydantic.networks import _BaseMultiHostUrl


class BaseDBConfig(BaseSettings):
    ...
    
    @field_validator('*')
    def format_config_from_dsn(cls, value: Any, info: ValidationInfo):
        if value is None:
            return {}

        if not isinstance(value, (str, MultiHostUrl, _BaseMultiHostUrl)):
            return value
    ....

but of course that's not an exhaustive solution.