On Dec 22, 2011, at 11:48 AM, peter sabaini wrote:
> Hey!
>
> This works for me -- almost :-)
>
> In my use case I need to override a column (to provide an artificial FK --
> some *erm old school mysql db) which seems to trigger SA into trying to
> reflect early
>
> Observe:
>
> from sqlalchemy import *
> from sqlalchemy.orm import *
> from sqlalchemy.ext.declarative import declarative_base, declared_attr
>
> class DeclarativeReflectedBase(object):
> _mapper_args = []
>
> @classmethod
> def __mapper_cls__(cls, *args, **kw):
> """Declarative will use this function in lieu of
> calling mapper() directly.
>
> Collect each series of arguments and invoke
> them when prepare() is called.
> """
>
> cls._mapper_args.append((args, kw))
>
> @classmethod
> def prepare(cls, engine):
> """Reflect all the tables and map !"""
> for args, kw in cls._mapper_args:
> klass = args[0]
> klass.__table__ = table = Table(
> klass.__tablename__,
> cls.metadata,
> autoload=True,
> autoload_with=engine)
> klass.__mapper__ = mapper(klass, table, **kw)
>
> @declared_attr
> def __table__(cls):
> """Return a placeholder to lull declarative into complacency"""
> return object()
>
> Base = declarative_base(cls=DeclarativeReflectedBase)
>
> class Foo(Base):
> __tablename__ = 'foo'
> quux = Column(String)
> bars = relationship("Bar")
>
> class Bar(Base):
> __tablename__ = 'bar'
>
>
yah well you want to not declare Column objects on the declared class directly
in this case because that trips off declarative into building up a Table, which
you don't have here, it only returns "object".
this could work really nicely with extend_existing, which has been enhanced in
0.7.4, but there seem to be some glitches preventing it from being super nice,
so I can't get that to work right now. Just send in those columns via your own
means:
class Base(object):
# ...
extra_cols = []
@classmethod
def prepare(cls, engine):
"""Reflect all the tables and map !"""
for args, kw in cls._mapper_args:
klass = args[0]
klass.__table__ = table = Table(
klass.__tablename__,
cls.metadata,
autoload=True,
autoload_with=engine,
*klass.extra_cols
)
klass.__mapper__ = mapper(klass, table, **kw)
class Bar(Base):
__tablename__ = 'bar'
extra_cols = [
Column('foo_id', Integer, ForeignKey('foo.id')),
]
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