Unexpected @field_validator behaviour

[rupprecht1993]

Bug

If you create a mandatory attribute for a custom class that derives from Model and create a instance of this class without providing the mandatory field, you do not get a Field required validation error and the custom validators are run (which would is the default behaviour for the BaseModel classes).

Current Behavior

import datetime
from typing import Optional

from odmantic import Field, Model
from pydantic import field_validator


class MyClass(Model):
    date_of_birth: datetime.datetime
    birth_year: Optional[int] = Field(None)

    @field_validator("birth_year", mode="before")
    def validate_birth_year(cls, v: int, values) -> int:
        """Validates the birth year."""
        if v is None:
            date_of_birth: datetime = values.data.get("date_of_birth")
            v = date_of_birth.year
        return v

running this returns

v = date_of_birth.year
        ^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'year'

Expected behavior

Expected would be the output that pydantic generates you if you derive directly from BaseModel without running the custom validators at all:

pydantic_core._pydantic_core.ValidationError: 1 validation error for MyClass
date_of_birth
  Field required [type=missing, input_value={}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.6/v/missing

Environment

• ODMantic version: 1.0.1
• Pydantic infos (output of python -c "import pydantic.utils; print(pydantic.utils.version_info())):

 pydantic version: 2.6.4
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=true
                 install path: /Users/lxr1046/Library/Caches/pypoetry/virtualenvs/clinical-etl-service-lM9J4W3w-py3.11/lib/python3.11/site-packages/pydantic
               python version: 3.11.8 (main, Feb  6 2024, 21:21:21) [Clang 15.0.0 (clang-1500.1.0.2.5)]
                     platform: macOS-14.4-arm64-arm-64bit
             related packages: fastapi-0.110.0 mypy-1.9.0 typing_extensions-4.10.0 pydantic-settings-2.2.1
                       commit: unknown