SQLAlchemy 声明式 + 跨多个不同数据库的关系

我花了一段时间，但我弄清楚了如何使用 SQLAlchemy 对两种不同类型的数据库之间的关系进行建模：

    Base = declarative_base()

    class Survey(Base):
        __tablename__ = 'SURVEY'
    
        survey_id = Column("SURVEY_ID", Integer, primary_key=True)
        term_id = Column("TERM_ID", Integer, nullable=False)
        
        # Because the TERM table is in Oracle, but the SURVEY table is in
        # MySQL, I can't rely on SQLAlchemy's ForeignKey.  Thus,
        # I need to specify the relationship entirely by hand, like so:
        term = relationship("Term",
            primaryjoin="Term.term_id==Survey.term_id",
            foreign_keys=[term_id],
            backref="surveys"
        )
        
    class Term(Base):
        __tablename__ = 'TERM'
        
        term_id   = Column(Integer, primary_key=True)
        term_name = Column(String(30))
        start_date = Column(Date)
        end_date = Column(Date)
    
    mysql_engine = create_engine(MYSQL)
    oracle_engine = create_engine(ORACLE)
    
    Session = scoped_session(sessionmaker(
        binds={
            Term: oracle_engine,
            Survey: mysql_engine
        }
    ))
    
    if __name__ == "__main__":
        survey = Session.query(Survey).filter_by(survey_id=8).one()
        print survey.term
        print survey.term.surveys

我必须这样做，因为 TERM 表位于 Oracle 数据库中，我对该数据库只有读取访问权限，并且我正在编写一个应用程序来记录学生针对该术语进行的调查。

上面的方法是有效的，但是当表的数量增加时，它非常脆弱，因为会话需要准确指定哪些映射类对应于哪个引擎。我真的很希望能够使用不同的Base定义哪些表属于哪个引擎，而不是单独绑定每个表。像这样：

    mysql_engine = create_engine(MYSQL)
    oracle_engine = create_engine(ORACLE)
        
    MySQLBase = declarative_base(bind=mysql_engine)
    OracleBase = declarative_base(bind=oracle_engine)
    
    class Survey(MySQLBase):
        __tablename__ = 'SURVEY'
    
        survey_id = Column("SURVEY_ID", Integer, primary_key=True)
        term_id = Column("TERM_ID", Integer, nullable=False)
        
    
    class Term(OracleBase):
        __tablename__ = 'ads_term_v'
        
        term_id   = Column(Integer, primary_key=True)
        term_name = Column(String(30))
        start_date = Column(Date)
        end_date = Column(Date)
    
    Survey.term = relationship("Term",
        primaryjoin="Term.term_id==Survey.term_id",
        foreign_keys=[Survey.term_id],
        backref="surveys"
    )
        
    Session = scoped_session(sessionmaker())
    
    if __name__ == "__main__":
        survey = Session.query(Survey).filter_by(survey_id=8).one()
        print survey.term
        print survey.term.surveys

不幸的是，当查询运行时，这会导致以下错误：

sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper|Survey|SURVEY, expression 'Term.term_id==Survey.term_id' failed to locate a name ("name 'Term' is not defined"). If this is a class name, consider adding this relationship() to the <class '__main__.Survey'> class after both dependent classes have been defined.

即使我did定义术语后，将关系（）添加到调查中。

有没有人有什么建议？

这个回复可能很晚了，但是您可以将元数据与声明性基础分开定义，然后将其传递给两者。 IE：

meta = MetaData()
mysql_engine = create_engine(MYSQL)
oracle_engine = create_engine(ORACLE)

MySQLBase = declarative_base(bind=mysql_engine, metadata=meta)
OracleBase = declarative_base(bind=oracle_engine, metadata=meta)