issues with pydantic dataclasses / sqlalchemy

test.py

from __future__ import annotations

from typing import TYPE_CHECKING

import pydantic.dataclasses

from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import MappedAsDataclass
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session


class Base(
    MappedAsDataclass,
    DeclarativeBase,
    dataclass_callable=pydantic.dataclasses.dataclass,
):
    if not TYPE_CHECKING:
        # workaround for problem 1 below
        id = Column(Integer, primary_key=True)


class B(Base):
    __tablename__ = "b"

    # problem 1 - there is no way to use a pydantic dataclass field
    # with init=False that keeps the value totally unset.  this field
    # fails validation.  if we add default=None to pass validation, SQLAlchemy
    # does not use the server side primary key generator.
    # SQLAlchemy will likely need to add a new default value DONT_SET to
    # work around this
    # id: Mapped[int | None] = mapped_column(primary_key=True, init=False)

    a_id: Mapped[int | None] = mapped_column(
        ForeignKey("a.id"), init=False, default=None
    )


class A(Base):
    __tablename__ = "a"

    # also problem 1 here
    # id: Mapped[int | None] = mapped_column(primary_key=True, init=False)

    data: Mapped[str]
    bs: Mapped[list[B]] = relationship("B")


e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
with Session(e) as s:
    # problem 2.  Pydantic lets the class init, then validators
    # come in and *rewrite* the collections, then assign them on __dict__.
    # patch that fixes this specific case here, however there can be many
    # more since pydantic writes into `__dict__` quite a lot
    a1 = A(data="a1", bs=[B(), B()])
    s.add(a1)
    s.commit()

Just tried and the following works on pydantic 2

from __future__ import annotations

import pydantic
import pydantic.dataclasses

from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import select
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import MappedAsDataclass
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session

assert pydantic.__version__ >= "2"

class Base(
    MappedAsDataclass,
    DeclarativeBase,
    dataclass_callable=pydantic.dataclasses.dataclass,
):
    pass

class B(Base):
    __tablename__ = "b"

    id: Mapped[int] = mapped_column(primary_key=True, init=False)

    a_id: Mapped[int | None] = mapped_column(
        ForeignKey("a.id"), init=False, default=None
    )

class A(Base):
    __tablename__ = "a"

    id: Mapped[int] = mapped_column(primary_key=True, init=False)

    data: Mapped[str]
    bs: Mapped[list[B]] = relationship("B")

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
with Session(e) as s:
    a1 = A(data="a1", bs=[B(), B()])
    s.add(a1)
    s.commit()

with Session(e) as s:
    a = s.scalars(select(A)).one()
    assert a.data == "a1"
    assert len(a.bs) == 2

i am working up a fix for issue number one on our end.

I guess we can just wait instead

how does the int id with init=False work? does it only validate arguments that were actually passed ? (because hooray if so?)

how does the int id with init=False work? does it only validate arguments that were actually passed ? (because hooray if so?)

no clue. I guess it does?

I think the best option at the moment is to wait for v2 to be released and at that point look if anything is still needed. At the moment is seems no, but it's a beta the current v2 version