[Sqlalchemy-tickets] Issue #4090: declared_attr.cascading doesn't work for __tablename__ (zzzeek/sqlalchemy)

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

New issue 4090: declared_attr.cascading doesn't work for __tablename__
https://bitbucket.org/zzzeek/sqlalchemy/issues/4090/declared_attrcascading-doesnt-work-for

David Lord:

I noticed this while fixing tablename generation in Flask-SQLAlchemy. The reason I use the metaclass instead of `declared_attr` is because the `cascading` option doesn't work for `__tablename__`. Instead, it's treated like a normal declared attr, where once an intermediate class sets a value, that's used going forward.

You also showed `declared_attr.cascading` for `__tablename__` in the docs for `__table_cls__`, which won't work as intended: https://gerrit.sqlalchemy.org/plugins/gitiles/zzzeek/sqlalchemy/+/refs/changes/27/527/2/doc/build/orm/extensions/declarative/api.rst#137

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

class Base:
    @declared_attr.cascading
    def __tablename__(cls):
        return cls.__name__.lower()

Base = declarative_base(cls=Base)

class User(Base):
    id = sa.Column(sa.Integer, primary_key=True)

class User2(User):
    __tablename__ = 'user2'
    id = sa.Column(sa.ForeignKey(User.id), primary_key=True)

class User3(User2):
    id = sa.Column(sa.ForeignKey(User2.id), primary_key=True)

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

```pytb
Traceback (most recent call last):
  File "/home/david/Projects/flask-sqlalchemy/example.py", line 23, in <module>
    class User3(User2):
  File "/home/david/.virtualenvs/flask-sqlalchemy/lib/python3.6/site-packages/sqlalchemy/ext/declarative/api.py", line 64, in __init__
    _as_declarative(cls, classname, cls.__dict__)
  File "/home/david/.virtualenvs/flask-sqlalchemy/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 88, in _as_declarative
    _MapperConfig.setup_mapping(cls, classname, dict_)
  File "/home/david/.virtualenvs/flask-sqlalchemy/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 103, in setup_mapping
    cfg_cls(cls_, classname, dict_)
  File "/home/david/.virtualenvs/flask-sqlalchemy/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 131, in __init__
    self._setup_table()
  File "/home/david/.virtualenvs/flask-sqlalchemy/lib/python3.6/site-packages/sqlalchemy/ext/declarative/base.py", line 395, in _setup_table
    **table_kw)
  File "/home/david/.virtualenvs/flask-sqlalchemy/lib/python3.6/site-packages/sqlalchemy/sql/schema.py", line 421, in __new__
    "existing Table object." % key)
sqlalchemy.exc.InvalidRequestError: Table 'user2' is already defined for this MetaData instance.  Specify 'extend_existing=True' to redefine options and columns on an existing Table object.
```