Flask專案裡面，如何同時使用MySQL和MongoDB，Sqlalchemy和MongoEngine？

from mongoengine import *  
import datetime

class Page(Document):  
    title = StringField(max_length=200, required=True)  
    date_modified = ateTimeField(default=datetime.datetime.now)
    author = xxxx????【这里怎么关联到User呢？？ 手工？】


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from sqlalchemy import Column, Integer, String
class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    fullname = Column(String)
    password = Column(String)