[Sqlalchemy-tickets] Issue #4091: declared_attr.cascading unconditionally sets attribute (zzzeek/sqlalchemy)

[David L. <iss...@bi...>]

New issue 4091: declared_attr.cascading unconditionally sets attribute
https://bitbucket.org/zzzeek/sqlalchemy/issues/4091/declared_attrcascading-unconditionally

David Lord:

I'm not sure if this is a bug or just a result of unclear documentation. I would expect that a cascading declared attr would not overwrite an attribute that's manually defined on a class, but that's not the case.

This makes it less useful for things like `__tablename__` (with #4090), where I would like the user to be able to set their own tablename to override the default.

```python3
import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base, declared_attr, \
    has_inherited_table

class Base:
    @declared_attr.cascading
    def id(cls):
        if has_inherited_table(cls):
            type = sa.ForeignKey(cls.__mro__[1].id)
        else:
            type = sa.Integer

        return sa.Column(type, primary_key=True)

Base = declarative_base(cls=Base)

class User(Base):
    __tablename__ = 'user'

class User2(User):
    __tablename__ = 'user2'
    # due to cascading, the created name is 'id' instead of 'user_id'
    id = sa.Column('user_id', sa.ForeignKey(User.id), primary_key=True)

engine = sa.create_engine('sqlite://', echo=True)
Base.metadata.create_all(engine)
```