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'
Gives:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/tmp/Python6244Mux.py", line 40, in <module>
class Foo(Base):
File
"/usr/lib/python2.6/site-packages/SQLAlchemy-0.7.4-py2.6-linux-x86_64.egg/sqlalchemy/ext/declarative.py",
line 1273, in __init__
_as_declarative(cls, classname, cls.__dict__)
File
"/usr/lib/python2.6/site-packages/SQLAlchemy-0.7.4-py2.6-linux-x86_64.egg/sqlalchemy/ext/declarative.py",
line 1177, in _as_declarative
if not table.c.contains_column(c):
AttributeError: 'object' object has no attribute 'c'
Hm, maybe I can try to add the column override later?
Thanks again
peter.
On Thu, Dec 22, 2011 at 4:41 PM, Michael Bayer <mike...@zzzcomputing.com>wrote:
>
> On Dec 22, 2011, at 9:37 AM, peter sabaini wrote:
>
> > Hey list,
> >
> > this sounds like it should be a FAQ, didn't find anything though:
> >
> > I want to use the ORM in a declarative style and have the table
> > definition reflected, eg sth like:
> >
> > class A(declarative_base()):
> > __tablename__ = 'A'
> > __table_args__ = {'autoload' : True}
> >
> > However to do this SA (quite reasonably, really) already needs an
> > engine. For various reasons I can only construct one after import time
> > however. Is there a way to do a kind of "late reflection", ie. have
> > the above class definition but trigger the reflection part later? I
> > could of course generate the class object later in a function when the
> > engine is already available but maybe there's something more
> > elegant...
>
> I was about to type up this recipe on the wiki and then most awesomely I
> already did it for someone ! hooray. The current technique for this is
> at
> http://www.sqlalchemy.org/trac/wiki/UsageRecipes/DeclarativeReflectedBase.
>
>
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