Thanks. We went with the helper class route, and it seems to be working
much better than attempting to use inheritance in a manner that seems
unsupported.
On Wednesday, September 6, 2023 at 1:24:04 PM UTC-5 Mike Bayer wrote:
> if you can't correct this model to apply the persistence details to the
> concrete class you wish to persist and query, then you'd do the suggested
> "enable_typechecks=False". There is no attribute in SQLAlchemy named
> "meta" and no stack trace is given here so I dont know to what that refers.
>
> Overall I'm not sure how this API_Person class is useful because you can't
> query for them. I would think that if you cant change the original
> model then you'd have this API_Person as a series of helper functions that
> accept a Person as their argument.
>
>
>
> On Wed, Sep 6, 2023, at 1:56 PM, 'Luna Lucadou' via sqlalchemy wrote:
>
> The project I am working on is split up into several modules. Previously,
> each module had its own ORM classes.
> However, due to several bugs arising from forgetting to update each
> module's ORM classes in lock step when adding new functionality, we have
> decided it would be best to extract the ORM classes which interact with our
> DB into their own module and make that available to the other modules via
> pip.
>
> One of these modules, which monitors for changes in an upstream DB and
> applies them to ours, has some methods which are not present in the other
> modules and which cannot be easily extracted out due to its dependencies on
> upstream DB functionality.
>
> As such, in this module, we must subclass the ORM models which interact
> with our DB:
>
> models.apimodels.db.person.py:
> #...
> @dataclass(init=False, eq=True, unsafe_hash=True)
> class Person(Base):
> __tablename__ = "person"
>
> id: Mapped[int] = mapped_column(primary_key=True)
> first_name: Mapped[str]
> last_name: Mapped[str]
> email_address: Mapped[str]
> office_address: Mapped[str]
> office_phone_number: Mapped[str]
>
> # ...
>
> etl.models.api_db.api_person.py:
> #...
> from apimodels.db.person import Person as PersonBase
> # ...
> class API_Person(PersonBase):
> __tablename__ = "person"
> __table_args__ = {"keep_existing": True}
>
> def get_pvi(self):
> # ...
>
> def get_historical_pvis(self) -> list[str]:
> # ...
>
> def __eq__(self):
> # ...
>
> def __hash__(self):
> # ...
>
> @staticmethod
> def from_upstream_hub_person(
> uh_person: Optional[UH_Person],
> ) -> Optional["API_Person"]:
> # ...
>
> Of note is that this subclass does not add any new attributes or modify
> existing ones, it merely adds some helper methods related to identifying
> primary key changes in the upstream DB. (This is also why we override the
> eq and hash methods provided by dataclasses - incoming changesets have to
> be matched against existing records, even when primary keys change
> upstream.)
>
> This is effectively single-table inheritance, but it is not a good fit for
> polymorphic_identity since it is not a distinct class, merely adding
> module-specific helper methods, and if I am reading the documentation
> correctly, using polymorphic_identity would mean any records touched by
> the API_Person subclass (which is all of them) would no longer be usable
> by other modules, which do not extend any of the models.
>
> From what I have read, it seems like the keep_existing param should be of
> use here, but neither it nor extend_existing set to True help with the
> errors I am seeing when attempting to interact with this subclass in any
> way:
>
> sqlalchemy.orm.exc.FlushError: Attempting to flush an item of type <class
> 'model.api_db.api_person.API_Person'> as a member of collection
> "Identifier.person". Expected an object of type <class
> 'apimodels.db.person.Person'> or a polymorphic subclass of this type. If
> <class 'model.api_db.api_person.API_Person'> is a subclass of <class
> 'apimodels.db.person.Person'>, configure mapper "Mapper[Person(person)]" to
> load this subtype polymorphically, or set enable_typechecks=False to allow
> any subtype to be accepted for flush.
>
> I did try setting enable_typechecks to False, but this results in a
> different error when attempting to use getattr on the subclass:
>
> AttributeError: 'Person' object has no attribute 'meta'
>
> Is there a better way of doing this?
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
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