It required us a while, however i determined using SQLAlchemy to model rapport across two different types of databases:

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

I must do that since the TERM table is within an Oracle database which I have only read access, and I am writing an application that records surveys, taken by students, about this term.

The above mentioned works, but it is very fragile when the amount of tables increases up, because the Session must specify which planned classes match which engine. I would love to have the ability to make use of a different Base to define which tables fit in with which engine, rather than binding each table individually. Such as this:

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

Regrettably, this leads to the next error once the query runs:

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.

despite the fact that I did add the connection() to Survey after Term was defined.

Does anybody have suggestions?

You cannot. AFAIK there is no single query against two different databases. Also, your Designs include to talk about exactly the same Metadata instance for use within the same query.

Possibly you are able to link the Oracle db towards the MySQL db around the DB layer via ODBC, then you'd only speak with MySQL. I have not carried this out, and I'm not sure how it operates.

You may also query both databases individually and filter and choose data around the application layer, whichever is less work.