Classical mapping, dataclasses with initialiser and filter clauses

Hi,

I have come across some unexpected behaviour when using dataclasses mapped to SQLAlchemy using classical mapping.

Here’s a snippet with the essence of the issue:

from __future__ import annotations

from dataclasses import dataclass, field

from sqlalchemy import Column, Table, String, Boolean
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import mapper, sessionmaker

Base = declarative_base()
metadata = Base.metadata

table = Table(
            "table",
            metadata,
            #
            Column("id", String, primary_key=True),
            Column("active", Boolean),
            )


@dataclass
class C:
    id: str
    active: bool = True


@dataclass
class C1:
    id: str
    active: bool


@dataclass
class C2:
    id: str
    active: bool = field(default_factory=lambda: True)


mapper(C, table)
mapper(C1, table)
mapper(C2, table)

Session = sessionmaker()
session = Session()

query = session.query(C)
print(query.filter(C.active == True))
print()

# Result (WRONG)
# SELECT "table".id AS table_id
# FROM "table"
# WHERE true

query = session.query(C1)
print(query.filter(C1.active == True))
print()

# Result: OK
# SELECT "table".id AS table_id, "table".active AS table_active
# FROM "table"
# WHERE "table".active = true

query = session.query(C2)
print(query.filter(C2.active == True))
print()

# Result: OK
# SELECT "table".id AS table_id, "table".active AS table_active
# FROM "table"
# WHERE "table".active = true

So, yes, I have found a workaround (using = field(default_factory=lambda: something) as an initializer instead of just = something), but it doesn’t look as good.

Is there some general advice regarding the interaction of dataclasses and SQLAlchemy ORM? (I have also tried with attrs, and got the same issue).

[keosak]

Sure, I can try over the weekend. I want to look over the dataclasses documentation and implementation more closely first, to make sure I’m not missing something. But then I’ll try to come up with the tests.

[zzzeek]

I didn’t like this idea at first but once I did it this might be a nicer way to go, it sure is tricky to set up these dataclasses though. See #5745 + the patch for complete POC it would be extremely helpful if you could test it out.