[Sqlalchemy-tickets] Issue #3847: declared_attr.cascading does not warn / error for non-mixin use (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 3847: declared_attr.cascading does not warn / error for non-mixin use
https://bitbucket.org/zzzeek/sqlalchemy/issues/3847/declared_attrcascading-does-not-warn-error

Michael Bayer:

use declared_attr.cascading on a non mixin, the "cascading" is ignored and it's only called once, totally confusing:

```
#!python


from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.declarative import declared_attr

Base = declarative_base()


class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)

    @declared_attr.cascading
    def foo(cls):
        if cls.__name__ == 'A':
            return Column(String)
        else:
            return Column(Integer)

class B(A):
    __tablename__ = 'b'
    id = Column(Integer, ForeignKey('a.id'), primary_key=True)


e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)


```

output:


```
#!python

CREATE TABLE a (
	id INTEGER NOT NULL, 
	foo VARCHAR, 
	PRIMARY KEY (id)
)


2016-11-07 20:35:49,091 INFO sqlalchemy.engine.base.Engine ()
2016-11-07 20:35:49,091 INFO sqlalchemy.engine.base.Engine COMMIT
2016-11-07 20:35:49,092 INFO sqlalchemy.engine.base.Engine 
CREATE TABLE b (
	id INTEGER NOT NULL, 
	PRIMARY KEY (id), 
	FOREIGN KEY(id) REFERENCES a (id)
)


```

this should ideally either work or raise an error, though for now a warning is likely the best we can do.