On Fri, Jul 29, 2022, at 6:57 PM, [email protected] wrote:
> Thank you, Mike!
>
> I’m still a little unclear: how would I use different *MetaData* for
> different sets of tables? For example, currently I use this function:
>
> *def create_base(schema: typing.Optional[str] = None) -> typing.Any:
> return declarative_base(
> cls=_Base,*
> * metadata=MetaData(schema=schema,
> naming_convention=_NAMING_CONVENTION),*
> * metaclass=_BaseMeta
> )*
> where I have a custom *_Base* (to supply e.g. useful *__str__* etc. helpers)
> and a custom *_BaseMeta* which derives from *DeclarativeBase*. The
> *MetaData*’s schema is a parameter to that function so that I can create
> different *Base* classes with their own *MetaData*. Using these different
> *Base* classes I then declare different table mappings, each with their own
> *MetaData*.
pep-484 typing is entirely not compatible with functions that return classes
unfortunately, so if you wanted Base to be recognized as a class, you have to
declare it as needed. The quickest way to do this is likely with the
as_declarative_base function:
mapper_registry = registry()
@mapper_registry.as_declarative_base()
class Base:
pass
Above, Base is now a new declarative base with its own MetaData collection.
>
> Or… would I now statically declare a class
> *CommonBase(metaclass=DeclarativeMeta)* (contains the helpers, etc.) and then
> derive specific classes with their respective *MetaData*, e.g.
>
> *class Base(CommonBase):*
> * metadata = MetaData(schema=schema, naming_convention=_NAMING_CONVENTION)*
>
> Or… maybe I’m thinking about this the wrong way?
right we have this in SQLAlchemy 2.0 called DeclarativeBase and it works
exactly that way. To get that in 1.4, here's one that I had to look in the
current source for DeclarativeMeta to figure out
class CommonMeta(DeclarativeMeta):
def __init__(cls, classname, bases, dict_, **kw):
if not bases or CommonBase in bases:
cls.registry = cls._sa_registry = reg = registry()
cls.metadata = reg.metadata
cls.__init__ = reg.constructor
type.__init__(cls, classname, bases, dict_)
else:
super().__init__(classname, bases, dict_, **kw)
class CommonBase(metaclass=CommonMeta):
pass
class Base(CommonBase):
pass
>
> Jens
>
>
> On Saturday, July 30, 2022 at 6:47:24 AM UTC+10 Mike Bayer wrote:
>>
>> yes use the recipe given at
>> https://docs.sqlalchemy.org/en/14/orm/declarative_styles.html#creating-an-explicit-base-non-dynamically-for-use-with-mypy-similar
>> . That is SQLAlchemy 1.4 documentation. SQLAlchemy 2.0, when released,
>> moves "declarative_base()" to legacy status for this reason and provides a
>> new base class to use.
>>
>
>
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> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
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