How to bind engine when I want, when using declarative_base in SQLAlchemy?

Question

Here's my code:

from sqlalchemy import create_engine, Column, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

database_url = 'mysql://some_path'
engine = create_engine(database_url)
Base = declarative_base(engine)

class Users(Base):
    __tablename__ = 'Users'
    __table_args__ = {'autoload':True}

metadata = Base.metadata
Session = sessionmaker(bind=engine)
session = Session()

It works, but...
Is it possible to bind engine when I want, not only at import time? So I can wrap this implementation into class.

For now, I get

    class Users(Base):
  File "/usr/lib/python2.5/site-packages/SQLAlchemy-0.6.5-py2.5.egg/sqlalchemy/ext/declarative.py", line 1231, in __init__
    _as_declarative(cls, classname, cls.__dict__)
  File "/usr/lib/python2.5/site-packages/SQLAlchemy-0.6.5-py2.5.egg/sqlalchemy/ext/declarative.py", line 1122, in _as_declarative
    **table_kw)
  File "/usr/lib/python2.5/site-packages/SQLAlchemy-0.6.5-py2.5.egg/sqlalchemy/schema.py", line 209, in __new__
    table._init(name, metadata, *args, **kw)
  File "/usr/lib/python2.5/site-packages/SQLAlchemy-0.6.5-py2.5.egg/sqlalchemy/schema.py", line 260, in _init
    msg="No engine is bound to this Table's MetaData. "
  File "/usr/lib/python2.5/site-packages/SQLAlchemy-0.6.5-py2.5.egg/sqlalchemy/schema.py", line 2598, in _bind_or_error
    raise exc.UnboundExecutionError(msg)
sqlalchemy.exc.UnboundExecutionError: No engine is bound to this Table's MetaData. Pass an engine to the Table via autoload_with=<someengine>, or associate the MetaData with an engine via metadata.bind=<someengine>

when engine is not specified: Base = declarative_base()

Answer 1

At least with SQLAlchemy 0.9 you can defer the binding by using DeferredReflection. See the example in the Using Reflection with Declarative section of the manual.

There, you can find the following example (simplified):

from sqlalchemy.ext.declarative import declarative_base, DeferredReflection

Base = declarative_base(cls=DeferredReflection)

class Foo(Base):
    __tablename__ = 'foo'
    bars = relationship("Bar")

class Bar(Base):
    __tablename__ = 'bar'

Base.prepare(e)

e is the engine.