Python + SqlAlchemy: Add Parent/Child Records When Both Parent & Child are New

Question

I\'ve asked a similar question already, but I think it would be much clearer if, rather than provide samples of what I\'ve already coded, I simply state the goal of the case I h

Answer 1

Based on the answers and comments from this question the following seems to work for me:

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

connection_url = "mssql+pyodbc://@localhost,49242/myDb?driver=ODBC+Driver+17+for+SQL+Server"
engine = create_engine(connection_url)

try:
    engine.execute(f"DROP TABLE person")
except:
    pass

Base = declarative_base()


class Person(Base):
    __tablename__ = 'person'

    id = Column(Integer, autoincrement=True, primary_key=True)
    name = Column(String(50))
    _parent_id = Column(Integer, ForeignKey('person.id'))
    parent = relation('Person', remote_side=[id])
    children = relation('Person', remote_side=[_parent_id], uselist=True)

    def __repr__(self):
        return f"<Person(id={self.id}, name='{self.name}'>)"


Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)
session = Session()

parent = Person(name='Homer')
child = Person(name='Bart')
parent.children.append(child)
child = Person(name='Lisa')
parent.children.append(child)

print(f"{parent.name}'s (unpersisted) children: {parent.children}")
# Homer's (unpersisted) children: [<Person(id=None, name='Bart'>), <Person(id=None, name='Lisa'>)]

session.add(parent)
session.commit()

print(f"{parent.name}'s (persisted) children: {parent.children}")
# Homer's (persisted) children: [<Person(id=2, name='Bart'>), <Person(id=3, name='Lisa'>)]

child = session.query(Person).filter(Person.name=='Lisa').first()
print(f"{child.name}'s parent: {child.parent}")
# Lisa's parent: <Person(id=1, name='Homer'>)

with engine.begin() as conn:
    print(conn.execute("SELECT * FROM person").fetchall())
    # [(1, 'Homer', None), (2, 'Bart', 1), (3, 'Lisa', 1)]