如何在SQLAlchemy中使用declarative_base时按需绑定引擎？

Question

这是我的代码：

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()

它可以运行，但...
我想问一下，能不能在我需要的时候绑定引擎，而不仅仅是在导入的时候？这样我就可以把这个实现放进一个类里。

现在，我得到的是

    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>

当没有指定引擎时：Base = declarative_base()

Answer 1

在SQLAlchemy 0.9版本中，你可以通过使用DeferredReflection来延迟绑定。你可以在手册的使用反射与声明性部分找到相关示例。

在那里，你可以找到以下示例（简化版）：

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是引擎。

Answer 2

不行，你不能在把autoload选项设置为True的情况下使用延迟绑定，因为SQLAlchemy在不知道数据库结构的情况下无法编译映射器。