This should work, right? This demonstrates an issue I'm having with composite primary keys.

composite_keys.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, ForeignKey, Integer, String, ForeignKeyConstraint
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, relationship

Base = declarative_base()

class Tenant(Base):
	__tablename__ = 'tenant'
	id = Column(Integer, primary_key=True)
	name = Column(String)

class Friend(Base):
	__tablename__ = 'friend'
	id = Column(Integer, primary_key=True)
	tenant_id = Column(ForeignKey('tenant.id'), primary_key=True)
	name = Column(String)
	tenant = relationship(Tenant)

class Parent(Base):
	__tablename__ = 'parent'
	id = Column(Integer, primary_key=True)
	tenant_id = Column(ForeignKey('tenant.id'), primary_key=True)
	tenant = relationship(Tenant)

class Child(Parent):
	__tablename__ = 'child'
	id = Column(Integer, primary_key=True)
	tenant_id = Column(ForeignKey('tenant.id'), primary_key=True)
	friend_id = Column(Integer)
	name = Column(String)

	__table_args__ = (
		# superclass relationship
		ForeignKeyConstraint(
			(id, tenant_id),
			('parent.id','parent.tenant_id'),
		),

		# friend relationship
		ForeignKeyConstraint(
			(friend_id, tenant_id),
			('friend.id','friend.tenant_id'),
		),
	)

	parent = relationship(Parent)
	friend = relationship(Friend)

engine = create_engine('sqlite:///:memory:', echo=True)
Session = sessionmaker(bind=engine)

def run():
	# Basic Setup
	Base.metadata.create_all(engine)
	session = Session()

	# Make a child with a friend
	tenant = Tenant(name="testing", id=1)
	child = Child(name="Bob", tenant=tenant, id=1)
	friend = Friend(name="George", tenant=tenant, id=1)
	child.friend = friend
	session.add_all([tenant, child, friend])
	session.commit()

	# Take away the friend
	child = session.query(Child).first()
	child.friend = None
	session.commit()

if __name__ == '__main__':
	run()

error

AssertionError: Dependency rule tried to blank-out primary key column 'child.tenant_id' on instance '<Child at 0x10c8719d0>'