tests failing due to possible conflict with SQLAlchemy

[cosmicpython]

tests failing due to possible conflict with SQLAlchemy

[hjwp]

i think i know what it is. i should probably add a footnote in the book. but you need to change the @dataclass(frozen=True) to @dataclass(unsafe_hash=True). because sqlalchemy reasons.

[jmaralc]

Sorry Harry, my bad. It was failing in a copy of the model file with the frozen=True that I generated before by following the chapter. Sorry again for the extra work and thanks, diving in the hashes topics of dataclasses gave me a nice insight of what is happening.

[nikoladsp]

Hi,
I hit the very same issue.

More to that, even with InstrumentationManager, dataclass is changed by SA: after mapping is performed, SA has added some of its internal attributes (_sa_instance_state and _sa_class_manager) to dataclass, so we can not speak about decoupling and this beats the purpose of Registry in the first place since dataclass only looks like its decoupled.

I am not sure if there is a clean way to do this with SA, and posted question here. Additionaly, turns out that InstrumentationManager is very old, untested and almost never used.

If anyone have some ideas, please share.

Thank you kindly

[hjwp]

as always it's a tradeoff. Yes, in order to work, SQLAlchemy has to get its claws pretty deep into your objects, so they are not "truly" decoupled. On the other hand, an ORM does a lot of the heavy lifting for getting objects in and out of database columns.

The alternative is not to use an ORM at all, which I've enjoyed doing on the last few projects. You have to write a lot of your own SQL tho! Check out the exercise for the reader here, so you can get a feel for the tradeoff http://www.cosmicpython.com/book/chapter_02_repository.html#api_endpoint_with_repo

[jmaralc]

Not sure about it. I'm taking the code from the repo and it is already defined as:

@dataclass(unsafe_hash=True)`
class OrderLine:
    orderid: str
    sku: str
    qty: int

[hjwp]

it's weird that you'd get a dataclasses.FrozenInstanceError then?

do you have a repo you can share?

[tomdottom]

After fighting SqlAlchemy and solving this before finding this issue I'd just like to say that you can use frozen dataclasses.
However, you will have to provide a custom instrumentation manager.

Example:

For a frozen OrderLine dataclass

@dataclass(unsafe_hash=True)
class OrderLine:
    orderid: str
    sku: str
    qty: int

We can use a custom FrozenDataclassInstrumentationManager .

from sqlalchemy import Column, Integer, MetaData String, Table
from sqlalchemy.ext.instrumentation import InstrumentationManager
from sqlalchemy.orm import mapper

metadata = MetaData()


DEL_ATTR = object()


class FrozenDataclassInstrumentationManager(InstrumentationManager):
    def install_member(self, class_, key, implementation):
        self.originals.setdefault(key, class_.__dict__.get(key, DEL_ATTR))
        setattr(class_, key, implementation)

    def uninstall_member(self, class_, key):
        original = self.originals.pop(key, None)
        if original is not DEL_ATTR:
            setattr(class_, key, original)
        else:
            delattr(class_, key)

    def dispose(self, class_):
        del self.originals
        delattr(class_, "_sa_class_manager")
    
    def manager_getter(self, class_):
        def get(cls):
            return cls.__dict__["_sa_class_manager"]
        return get

    def manage(self, class_, manager):
        self.originals = {}
        setattr(class_, "_sa_class_manager", manager)

    def get_instance_dict(self, class_, instance):
        return instance.__dict__

    def install_state(self, class_, instance, state):
        instance.__dict__["state"] = state

    def remove_state(self, class_, instance, state):
        del instance.__dict__["state"]

    def state_getter(self, class_):
        def find(instance):
            return instance.__dict__["state"]
        return find


OrderLine.__sa_instrumentation_manager__ = FrozenDataclassInstrumentationManager


orderline_table = Table(
    "order_lines",
    metadata,
    Column("id", Integer, primary_key=True, autoincrement=True),
    Column("sku", String(128), nullable=False),
    Column("quantity", Integer, nullable=False),
)

mapper(
    OrderLine,
    orderline_table,
)

https://stackoverflow.com/a/66941711/2398354