[Sqlalchemy-tickets] Issue #3282: enhance use_alter to be more forgiving, include an "auto" mode (zzzeek/sqlalchemy)

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

New issue 3282: enhance use_alter to be more forgiving, include an "auto" mode
https://bitbucket.org/zzzeek/sqlalchemy/issue/3282/enhance-use_alter-to-be-more-forgiving

Mike Bayer:

the case of a schema that's been reflected, or otherwise incurs dependencies, is too dependent on simplistic use_alter flags in order to do subsequent schema creates.  introduce an "auto" mode that figures this out more naturally at metadata create time, and just knows how to warn/error in the drop case if a name is missing.  allow use_alter to be meaningfully changed after the fact as well.

```
#!python

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

Base = declarative_base()

class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)
    b_id = Column(ForeignKey('b.id'))
    bs = relationship("B")

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

from sqlalchemy.schema import AddConstraint, DropConstraint
from sqlalchemy import event


for table in Base.metadata.tables.values():

    for fkc in table.foreign_keys:
        # setting this here isn't good enough, because
        # we do the events as a fixed thing inside of set_parent().
        fkc.use_alter = True

        # we don't actually need a name for the CREATE part, just
        # the drop.
        fkc.constraint.name = table.name

        event.listen(Base.metadata, "after_create", AddConstraint(fkc.constraint))
        event.listen(Base.metadata, "before_drop", DropConstraint(fkc.constraint))

e = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
#Base.metadata.drop_all(e)
Base.metadata.create_all(e)
```