Discriminated Unions Raise "Cannot have a (non-optional) union as a SQLAlchemy field" (Regression?)

[danbreu]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the SQLModel documentation, with the integrated search.
• I already searched in Google "How to X in SQLModel" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to SQLModel but to Pydantic.
• I already checked if it is not related to SQLModel but to SQLAlchemy.

Commit to Help

• I commit to help with one of those options 👆

Example Code

from typing import Union

from sqlmodel import Field, SQLModel
from typing_extensions import Literal


class Cat(SQLModel):
    pet_type: Literal["cat"]
    meows: int


class Dog(SQLModel):
    pet_type: Literal["dog"]
    barks: float


class Lizard(SQLModel):
    pet_type: Literal["reptile", "lizard"]
    scales: bool


class Model(SQLModel, table=True):
    pet: Union[Cat, Dog, Lizard] = Field(..., discriminator="pet_type")
    n: int

Description

Creating a model containing a discriminated union, as shown in the working example in the last comment in #356, raises the exception below.
The only change to the example code is adding table=True to Model.

Traceback (most recent call last):
  File "/data/tool-chain/test.py", line 22, in <module>
    class Model(SQLModel, table=True):
  File "/usr/local/lib/python3.10/dist-packages/sqlmodel/main.py", line 478, in __new__
    col = get_column_from_field(v)
  File "/usr/local/lib/python3.10/dist-packages/sqlmodel/main.py", line 623, in get_column_from_field
    sa_type = get_sqlalchemy_type(field)
  File "/usr/local/lib/python3.10/dist-packages/sqlmodel/main.py", line 567, in get_sqlalchemy_type
    type_ = get_type_from_field(field)
  File "/usr/local/lib/python3.10/dist-packages/sqlmodel/_compat.py", line 191, in get_type_from_field
    raise ValueError(
ValueError: Cannot have a (non-optional) union as a SQLAlchemy field

Operating System

Linux

Operating System Details

No response

SQLModel Version

0.0.19

Python Version

3.10.12

Additional Context

Pydantic version: 2.8.2
Sqlalchemy version: 2.0.31

[danbreu]

This question was based on a misunderstanding of how SQLModel works. There is no way for SQLModel to automatically "collapse" nested models into a top-level table model.
And, as far as I understand, discriminated unions are only available for the Pydantic-based (i.e. non-ORM) part of SQLModel.