Problems with Update and Table Inheritance with SQLAlchemy

I'm building an inheritance table schema like the following:

Spec Code

class Person(Base):
    __tablename__ = 'people'
    id = Column(Integer, primary_key=True)
    discriminator = Column('type', String(50))
    updated = Column(DateTime, server_default=func.now(), onupdate=func.now())
    name = Column(String(50))
    __mapper_args__ = {'polymorphic_on': discriminator}

class Engineer(Person):
    __mapper_args__ = {'polymorphic_identity': 'engineer'}
    start_date = Column(DateTime)

class Manager(Person):
    __mapper_args__ = {'polymorphic_identity': 'manager'}
    start_date = Column(DateTime)

UPDATED (WORKING) CODE

import os
import sys

from sqlalchemy import Column, create_engine, ForeignKey, Integer, String, DateTime

from sqlalchemy.orm import sessionmaker
from sqlalchemy.sql import func
from sqlalchemy.ext.declarative import declarative_base


try:
   os.remove('test.db')
except FileNotFoundError:
   pass 

engine = create_engine('sqlite:///test.db', echo=True)
Session = sessionmaker(engine)

Base = declarative_base()


class People(Base):
    __tablename__ = 'people'
    discriminator = Column('type', String(50))
    __mapper_args__ = {'polymorphic_on': discriminator}

    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    updated = Column(DateTime, server_default=func.now(), onupdate=func.now())

class Engineer(People):
    __tablename__ = 'engineer'
    __mapper_args__ = {'polymorphic_identity': 'engineer'}
    id = Column(Integer, ForeignKey('people.id'), primary_key=True)
    kind = Column(String(100), nullable=True)

Base.metadata.create_all(engine)

session = Session()

e = Engineer()
e.name = 'Mike'
session.add(e)
session.flush()
session.commit()

# works when updating the object
e.name = "Doug"
session.add(e)
session.commit()


# works using the base class for the query
count = session.query(People).filter(
                           People.name.is_('Doug')).update({People.name: 'James'})

# fails when using the derived class
count = session.query(Engineer).filter(
                           Engineer.name.is_('James')).update({Engineer.name: 'Mary'})

session.commit()
print("Count: {}".format(count))

Note: this is slightly modified example from sql docs

If I try to update the name for Engineer two things should happen.

1. update statement to the People table on column name
2. automatic trigger of update to the updated column on the People table

For now, i'd like to focus on number 1. Things like the example below (as also documented in the full code example) will result in invalid SQL

session.query(Engineer).filter(
                           Engineer.name.is_('James')).update({Engineer.name: 'Mary'})

I believe the above generates the following:

UPDATE engineer SET name=?, updated=CURRENT_TIMESTAMP FROM people WHERE people.name IS ?

Again, this is invalid. The statement is trying to update rows in incorrect table. name is in the base table.

I'm a little unclear about how inheritance tables should work but it seems like updates should work transparently with the derived object. Meaning, when I update Engineer.name querying against the Engineer object SQLAlchemy should know to update the People table. To complicate things a bit more, what happens if I try to update columns which exist in two tables

session.query(Engineer).filter(
                           Engineer.name.is_('James')).update({Engineer.name: 'Mary', Engineer.start_date: '1997-01-01'})

I suspect SQLAlchemy will not issue two update statements.