SQLALCHEMY deleting Association Object does not delete it from the parent and child

I am trying to delete an association object. It is deleted from the table but the parent object still has the child object in its collection. Here is my Code:

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

engine = sqlalchemy.create_engine('sqlite:///:memory:')
Base = declarative_base()


class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Association", back_populates="parent")

    def __repr__(self) -> str:
        return f"Parent: {self.id}, {self.children}"


class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parents = relationship("Association", back_populates="child")

    def __repr__(self) -> str:
        return f"Child: {self.id}, {self.parents}"


class Association(Base):
    __tablename__ = 'association'
    parent_id = Column(Integer, ForeignKey('parent.id'), primary_key=True)
    child_id = Column(Integer, ForeignKey('child.id'), primary_key=True)
    child = relationship("Child", back_populates="parents")
    parent = relationship("Parent", back_populates="children")

    def __repr__(self) -> str:
        return f"Association: ({self.parent}); ({self.child};)"


Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine, autoflush=False, autocommit=False, expire_on_commit=False)
session = Session()

p = Parent()
c = Child()

session.add(p)
session.add(c)
session.commit()

association = Association(parent=p, child=c)
session.add(association)
session.commit()

assert session.query(Association).all()
assert session.query(Parent).all()[0].children
assert session.query(Child).all()[0].parents


session.delete(association)
session.commit()

assert not session.query(Association).all()
assert not session.query(Parent).all()[0].children
assert not session.query(Child).all()[0].parents

Now when I delete an association object and query it from the table association it's gone. However if I query the parent or child object the association is still in their collection field. How can I configure it, so when I delete the association object the object is also removed completely from the Parent collection and the Child collection.