sqlalchemy: Create Generated Column in MariaDB cannot specify null

Discussed in https://github.com/sqlalchemy/sqlalchemy/discussions/10055

<div type='discussions-op-text'>

^{Originally posted by iamrinshibuya July 3, 2023} Hello, when using Computed Columns, the following code works without any changes on PostgreSQL, but it fails in MariaDB.

import asyncio

from sqlalchemy import Computed, text
from sqlalchemy.orm import (
    Mapped,
    DeclarativeBase,
    mapped_column,
)
from sqlalchemy.ext.asyncio import create_async_engine

# works with this
# engine = create_async_engine('postgresql+psycopg://...')

# does not work with this
engine = create_async_engine('mariadb+asyncmy://...')

class Base(DeclarativeBase):
    pass

class Sqaure(Base):
    __tablename__ = 'square'
    id: Mapped[int] = mapped_column(primary_key=True)
    side: Mapped[int]
    area: Mapped[int] = mapped_column(Computed(text('4 * side')), index=True)

async def main():
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.drop_all)
        await conn.run_sync(Base.metadata.create_all)

asyncio.run(main())

sqlalchemy.exc.ProgrammingError: (asyncmy.errors.ProgrammingError) (1064, "You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'NOT NULL, \n\tPRIMARY KEY (id)\n)' at line 4")
[SQL: 
CREATE TABLE square (
        id INTEGER NOT NULL AUTO_INCREMENT, 
        side INTEGER NOT NULL, 
        area INTEGER GENERATED ALWAYS AS (4 * side) NOT NULL, 
        PRIMARY KEY (id)
)

]
(Background on this error at: https://sqlalche.me/e/20/f405)

What should I do so that the table gets created in MariaDB?

Making the column nullable (area: Mapped[int | None]) seems to fix this, but I have the following concerns

• I find it counterproductive in my case, the generated column always has a value.
• In PostgreSQL this is a “non-nullable” / required column and I’d like to not change the behavior just to accommodate MariaDB (the code has to support both dialects)</div>

The docs of mariadb seem to indicate that the null clause is not allowed with generated always https://mariadb.com/kb/en/generated-columns/