SQLAlchemy: добавление объектов в session

class Student(abstract.Base, abstract.Model):
    id = mapped_column(sqltypes.Integer(), primary_key=True)
    ...
    homeworks = relationship('Homework', back_populates='student', lazy='selectin')
 
class Homework(abstract.Base, abstract.Model):
    id = mapped_column(sqltypes.Integer(), primary_key=True)
    ...
    student_id = mapped_column(schema.ForeignKey('students.id'), nullable=False)
    student = relationship('Student', back_populates='homeworks')
    tasks = relationship('Task', back_populates='homework', lazy='selectin')
 
class Exercise(abstract.Base, abstract.Model):
    id = mapped_column(sqltypes.Integer(), primary_key=True)
    ...
    tasks = relationship('Task', back_populates='exercise', lazy='selectin')
 
class Task(abstract.Base, abstract.Model):
    id = mapped_column(sqltypes.Integer(), primary_key=True)
    ...
    exercise_id = mapped_column(schema.ForeignKey('exercises.id'), nullable=False)
    homework_id = mapped_column(schema.ForeignKey('homeworks.id'), nullable=False)
    exercise = relationship('Exercise', back_populates='tasks')
    homework = relationship('Homework', back_populates='tasks')

class PersistenceManager:
    __instance: Self = None
 
    def __new__(cls, *args, **kwargs):
        if cls.__instance is None:
            cls.__instance = super().__new__(cls)
            cls.__instance.__init__(*args, **kwargs)
        return cls.__instance
 
    _default_models_config: dict = utils.CONFIG['models-database']
 
    def __init__(self, models_config: dict = None):
        if not models_config:
            models_config = self._default_models_config
        self.models: Engine = create_engine(URL.create(**models_config))
 
    def write_model(self, obj):
        with Session(self.models) as session:
            session.add(obj)
            try:
                session.commit()
            except IntegrityError as exc:
                if 'UNIQUE' not in str(exc):
                    print(exc)
 
    def read_model(self, model: abstract.Model, primary_key: str, value):
        with Session(self.models) as session:
            stmt = select(model).where(getattr(model, primary_key) == value)
            result = session.execute(stmt).scalar()
        if not result:
            raise exceptions.NoResultsError()
        return result