sqlalchemy 2.0 and ABCMeta

[Iwan Vosloo]

Hi there,

We are migrating our code from SqlAlchemy 1.4 to 2.0 (2.0.23 to be
specific).

We have had the following, which allowed some classes inheriting from
our Base to use an ABCMeta metaclass:

-----------------------------------------------
class DeclarativeABCMeta(DeclarativeMeta, ABCMeta):
pass

metadata = MetaData(naming_convention=naming_convention)
Base = declarative_base(metadata=metadata, metaclass=DeclarativeABCMeta)
-----------------------------------------------

The code above works fine on 2.0, but if we want heed the 2.0 docs that
comment that declarative_base is superceded by using a class inheriting
from DeclarativeBase, we ought to have something like:

-----------------------------------------------
class DeclarativeABCMeta(DeclarativeMeta, ABCMeta):
pass

metadata = MetaData(naming_convention=naming_convention)

class Base(DeclarativeBase, metaclass=DeclarativeABCMeta):
"""A Base for using with declarative."""
__abstract__ = True
metadata = metadata
-----------------------------------------------

This, however breaks when it hits the first class inheriting from Base:

-----------------------------------------------
class SchemaVersion(Base):
__tablename__ = 'reahl_schema_version'
id = Column(Integer, primary_key=True)
version = Column(String(50))
egg_name = Column(String(80))
-----------------------------------------------

With:

[site-packages]/sqlalchemy/orm/decl_api.py:195: in __init__
_as_declarative(reg, cls, dict_)
[site-packages]/sqlalchemy/orm/decl_base.py:247: in _as_declarative
return _MapperConfig.setup_mapping(registry, cls, dict_, None, {})
[site-packages]/sqlalchemy/orm/decl_base.py:328: in setup_mapping
return _ClassScanMapperConfig(
[site-packages]/sqlalchemy/orm/decl_base.py:520: in __init__
super().__init__(registry, cls_, mapper_kw)
[site-packages]/sqlalchemy/orm/decl_base.py:344: in __init__
instrumentation.register_class(
[site-packages]/sqlalchemy/orm/instrumentation.py:684: in register_class
manager._update_state(
[site-packages]/sqlalchemy/orm/instrumentation.py:209: in _update_state
registry._add_manager(self)
[site-packages]/sqlalchemy/orm/decl_api.py:1380: in _add_manager
raise exc.ArgumentError(
E sqlalchemy.exc.ArgumentError: Class '<class
'reahl.sqlalchemysupport.sqlalchemysupport.SchemaVersion'>' already has
a primary mapper defined.

Any ideas on what we are doing wrong here?

Thanks
Iwan

--

[Mike Bayer]

hi -

I'm not sure what the issue is but if you are tinkering with metaclasses, we first off have an alternate version of DeclarativeBase called DeclarativeBaseNoMeta that has no metaclass installed, and may be a better place to build off custom metaclass solutions as you wont need to be subclassing the internal metaclass used by DeclarativeBase.

I would advise reading the source code for the DeclarativeBase / DeclarativeBaseNoMeta classes, paying special attention to the __init_subclass__() method which is where Declarative now does its work, and ensuring any __init_subclass__ method on your own class is working as expected and not interfering with the one called by SQLAlchemy's class (or just vendor the method yourself if necessary).

> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
> ---
> You received this message because you are subscribed to the Google
> Groups "sqlalchemy" group.
> To unsubscribe from this group and stop receiving emails from it, send
> an email to sqlalchemy+...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/sqlalchemy/5d616bf0-ffa7-4061-adaf-cf1c7577e0fc%40reahl.org.