How to convert SQLAlchemy model object to pure python object

I have User(db.Model) and Photo(db.Model) and one to many relation between them, thus user has many photos.

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    photos = db.relationship('Photo', backref='user', lazy='dynamic')
    def to_json(self):
        json = {
            'id': self.id,
            'photos': [photo.to_json() for photo in self.photos] if self.photos else None,
            'name': self.name
        }
        return json
class Photo(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    name = db.Column(db.String(1024), nullable=False)

    def to_json(self):
        json = {
            'id': self.id,
            'user_id': self.user_id,
            'name': self.name
        }
        return json

My goal was to select all of the users with all their photos with single query, and I did it using native query:

from sqlalchemy import text
statement = text('SELECT u.*, p.* FROM user u LEFT JOIN photo p ON u.id=p.user_id')
user_photo_query = db.session.query(User, Photo).from_statement(statement)
user_photo_tuples = user_photo_query.all()
users_dict = {}
db.session.expunge_all()
db.session.close()
for u, p in user_photo_tuples:
    if not users_dict.get(u.id, None):
        u.photos = list()
        users_dict.update({u.id: u})
    users_dict.get(u.id).photos.append(p)
users = users_dict.values()
return jsonify({'users': [user.to_json() for user in users]})

In the result of this query there is list of tuples (User_240298428, Photo_20394823). The photo is different in every record of this list. The user is the same for some records of the list. Because I don't want the users and the photos object to be bound to the session anymore I do:

db.session.expunge_all()
db.session.close()

So I iterate through the list and try to add all photos in the corresponding user.photos list.

users_dict.get(u.id).photos.append(p)

Here I get error:

Parent instance is not bound to a Session; lazy load operation of attribute 'user' cannot proceed

This is because of the relation that the user object has to the photo object:

photos = db.relationship('Photo', backref='user', lazy='dynamic')

What I want is to use user.photos as regular list, and not as SQLAlchemy relation. I want to add photos to the user list without this being connected somehow to sqlalchemy. Hope it's clear