Sqlalchemy Alembic迁移：如何删除多态标识

我有以下设置

class Content(Base):
    """Content object"""
    __tablename__ = "content"
    id = Column(Integer, primary_key=True)
    name = Column(Unicode(255),unique=True, nullable=False)
    title = Column(Unicode(255))
    body = Column(UnicodeText)
    created = Column(DateTime, default=func.now())
    modified = Column(DateTime, onupdate=func.now())
    type = Column(String(20))
    __mapper_args__ = {
    'polymorphic_on':type,
    'polymorphic_identity':'content',
    'with_polymorphic':'*'
}


class Locality(Content):
    __tablename__ = "local"
    id = Column(Integer, ForeignKey('content.id'),primary_key=True)

    city_name = Column(Unicode(80))
    __mapper_args__ = {'polymorphic_identity':'city'}

现在，我使用alembic删除了Location表。 每次查询内容时，我都会得到

AssertionError: No such polymorphic_identity 'city' is defined

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我如何删除这个多态性标识

我通过MySQL控制台对内容应用MySQL“delete from”命令，找到类型为“city”的内容，从而克服了这个问题

delete from content where content.type='city';