Failure creating tables with Annotated[None,...] in unions

[nkrishnaswami]

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Example Code

from typing import Annotated, ClassVar

from sqlalchemy import create_engine
from sqlmodel import Field, Relationship, SQLModel


class Child(SQLModel, table=True):
    __tablename__: ClassVar[str] = 'child'

    id: str = Field(primary_key=True)


class Parent(SQLModel, table=True):
    __tablename__: ClassVar[str] = 'parent'

    id: str = Field(primary_key=True)
    child: Child | Annotated[None, 1] = Relationship()


SQLModel.metadata.create_all(create_engine('sqlite://'))

Description

I am trying to serve SQLModel models via FastAPI. However, when a field has type Optional[TheType], the OpenAPI schema generated uses an anyOf TheType or null (field with value either TheType or an explicit null), which isn’t what I meant and is somewhat less usefully consumed by OpenAPI tooling, e.g. no drop-down for enumerations in the docs.
I tried mitigating this by switching from Optional[TheType] to TheType | SkipJsonSchema[None], ie using SkipJsonSchema from the pydantic.json_schema package (importantly, equivalent to typing.Annotated[NoneType, SkipJsonSchema()]). But after that, SQLModel.metadata.create_all started to fail in two different ways:

1. when the field is a relationship with another table, it fails with ValueError: Cannot have a Union of None and None as a SQLAlchemy field in (SQLModel's) _compat.py:get_relationship_to. (This is surprising since I’d expect both of the union members to be is not NoneType; e.g., args of (int, typing.Annotated[NoneType, 1]); in fact it appears to be a missing elif in the pydantic v2 get_relationship_to union branch.).
2. when using a non-relationship field (such as a float), it fails with ValueError: Cannot have a (non-optional) union as a SQLlchemy field in _compat.py:get_type_from_field. (This is less surprising since I’d expect both union members to be is not NoneType, so the union would fail the one-must-be-NoneType test. This function does not check for both NoneType)

This is perhaps a bit niche, but would there be interest in adding support for annotated NoneType/accepting a PR? Thanks!

Operating System

macOS

Operating System Details

No response

SQLModel Version

0.0.16

Python Version

Python 3.11.7

Additional Context

The alternate (non-relationship) testcase:

from typing import Annotated, ClassVar

from sqlalchemy import create_engine
from sqlmodel import Field, SQLModel


class Table(SQLModel, table=True):
    __tablename__: ClassVar[str] = 'table'

    id: str = Field(primary_key=True)
    field: float | Annotated[None, 1]


SQLModel.metadata.create_all(create_engine('sqlite://'))