from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    data = Column(String)
    bs = relationship("B", back_populates="a")


class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey("a.id"))
    data = Column(String)
    a = relationship("A", back_populates="bs")

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)

b1 = B()
a1 = A(bs=[b1])
s.add_all([a1, b1])
s.commit()

b1.a
del b1.a

s.commit()

assert b1.a is None