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Remove Relationships

Now let's say that Spider-Boy tells Rusty-Man something like:

I don't feel so good Mr. Sharp

And then for some reason needs to leave the Preventers for some years. ๐Ÿ˜ญ

We can remove the relationship by setting it to None, the same as with the team_id, it also works with the new relationship attribute .team:

# Code above omitted ๐Ÿ‘†

def update_heroes():
    with Session(engine) as session:
        statement = select(Hero).where(Hero.name == "Spider-Boy")
        result = session.exec(statement)
        hero_spider_boy = result.one()

        hero_spider_boy.team = None
        session.add(hero_spider_boy)
        session.commit()

        session.refresh(hero_spider_boy)
        print("Spider-Boy without team:", hero_spider_boy)

# Code below omitted ๐Ÿ‘‡
๐Ÿ‘€ Full file preview
from typing import List, Optional

from sqlmodel import Field, Relationship, Session, SQLModel, create_engine, select


class Team(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    headquarters: str

    heroes: List["Hero"] = Relationship(back_populates="team")


class Hero(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    secret_name: str
    age: Optional[int] = None

    team_id: Optional[int] = Field(default=None, foreign_key="team.id")
    team: Optional[Team] = Relationship(back_populates="heroes")


sqlite_file_name = "database.db"
sqlite_url = f"sqlite:///{sqlite_file_name}"

engine = create_engine(sqlite_url, echo=True)


def create_db_and_tables():
    SQLModel.metadata.create_all(engine)


def create_heroes():
    with Session(engine) as session:
        team_preventers = Team(name="Preventers", headquarters="Sharp Tower")
        team_z_force = Team(name="Z-Force", headquarters="Sister Margaretโ€™s Bar")

        hero_deadpond = Hero(
            name="Deadpond", secret_name="Dive Wilson", team=team_z_force
        )
        hero_rusty_man = Hero(
            name="Rusty-Man", secret_name="Tommy Sharp", age=48, team=team_preventers
        )
        hero_spider_boy = Hero(name="Spider-Boy", secret_name="Pedro Parqueador")
        session.add(hero_deadpond)
        session.add(hero_rusty_man)
        session.add(hero_spider_boy)
        session.commit()

        session.refresh(hero_deadpond)
        session.refresh(hero_rusty_man)
        session.refresh(hero_spider_boy)

        print("Created hero:", hero_deadpond)
        print("Created hero:", hero_rusty_man)
        print("Created hero:", hero_spider_boy)

        hero_spider_boy.team = team_preventers
        session.add(hero_spider_boy)
        session.commit()
        session.refresh(hero_spider_boy)
        print("Updated hero:", hero_spider_boy)

        hero_black_lion = Hero(name="Black Lion", secret_name="Trevor Challa", age=35)
        hero_sure_e = Hero(name="Princess Sure-E", secret_name="Sure-E")
        team_wakaland = Team(
            name="Wakaland",
            headquarters="Wakaland Capital City",
            heroes=[hero_black_lion, hero_sure_e],
        )
        session.add(team_wakaland)
        session.commit()
        session.refresh(team_wakaland)
        print("Team Wakaland:", team_wakaland)

        hero_tarantula = Hero(name="Tarantula", secret_name="Natalia Roman-on", age=32)
        hero_dr_weird = Hero(name="Dr. Weird", secret_name="Steve Weird", age=36)
        hero_cap = Hero(
            name="Captain North America", secret_name="Esteban Rogelios", age=93
        )

        team_preventers.heroes.append(hero_tarantula)
        team_preventers.heroes.append(hero_dr_weird)
        team_preventers.heroes.append(hero_cap)
        session.add(team_preventers)
        session.commit()
        session.refresh(hero_tarantula)
        session.refresh(hero_dr_weird)
        session.refresh(hero_cap)
        print("Preventers new hero:", hero_tarantula)
        print("Preventers new hero:", hero_dr_weird)
        print("Preventers new hero:", hero_cap)


def select_heroes():
    with Session(engine) as session:
        statement = select(Team).where(Team.name == "Preventers")
        result = session.exec(statement)
        team_preventers = result.one()

        print("Preventers heroes:", team_preventers.heroes)


def update_heroes():
    with Session(engine) as session:
        statement = select(Hero).where(Hero.name == "Spider-Boy")
        result = session.exec(statement)
        hero_spider_boy = result.one()

        hero_spider_boy.team = None
        session.add(hero_spider_boy)
        session.commit()

        session.refresh(hero_spider_boy)
        print("Spider-Boy without team:", hero_spider_boy)


def main():
    create_db_and_tables()
    create_heroes()
    select_heroes()
    update_heroes()


if __name__ == "__main__":
    main()

And of course, we should remember to add this update_heroes() function to main() so that it runs when we call this program from the command line:

# Code above omitted ๐Ÿ‘†

def main():
    create_db_and_tables()
    create_heroes()
    select_heroes()
    update_heroes()

# Code below omitted ๐Ÿ‘‡
๐Ÿ‘€ Full file preview
from typing import List, Optional

from sqlmodel import Field, Relationship, Session, SQLModel, create_engine, select


class Team(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    headquarters: str

    heroes: List["Hero"] = Relationship(back_populates="team")


class Hero(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    secret_name: str
    age: Optional[int] = None

    team_id: Optional[int] = Field(default=None, foreign_key="team.id")
    team: Optional[Team] = Relationship(back_populates="heroes")


sqlite_file_name = "database.db"
sqlite_url = f"sqlite:///{sqlite_file_name}"

engine = create_engine(sqlite_url, echo=True)


def create_db_and_tables():
    SQLModel.metadata.create_all(engine)


def create_heroes():
    with Session(engine) as session:
        team_preventers = Team(name="Preventers", headquarters="Sharp Tower")
        team_z_force = Team(name="Z-Force", headquarters="Sister Margaretโ€™s Bar")

        hero_deadpond = Hero(
            name="Deadpond", secret_name="Dive Wilson", team=team_z_force
        )
        hero_rusty_man = Hero(
            name="Rusty-Man", secret_name="Tommy Sharp", age=48, team=team_preventers
        )
        hero_spider_boy = Hero(name="Spider-Boy", secret_name="Pedro Parqueador")
        session.add(hero_deadpond)
        session.add(hero_rusty_man)
        session.add(hero_spider_boy)
        session.commit()

        session.refresh(hero_deadpond)
        session.refresh(hero_rusty_man)
        session.refresh(hero_spider_boy)

        print("Created hero:", hero_deadpond)
        print("Created hero:", hero_rusty_man)
        print("Created hero:", hero_spider_boy)

        hero_spider_boy.team = team_preventers
        session.add(hero_spider_boy)
        session.commit()
        session.refresh(hero_spider_boy)
        print("Updated hero:", hero_spider_boy)

        hero_black_lion = Hero(name="Black Lion", secret_name="Trevor Challa", age=35)
        hero_sure_e = Hero(name="Princess Sure-E", secret_name="Sure-E")
        team_wakaland = Team(
            name="Wakaland",
            headquarters="Wakaland Capital City",
            heroes=[hero_black_lion, hero_sure_e],
        )
        session.add(team_wakaland)
        session.commit()
        session.refresh(team_wakaland)
        print("Team Wakaland:", team_wakaland)

        hero_tarantula = Hero(name="Tarantula", secret_name="Natalia Roman-on", age=32)
        hero_dr_weird = Hero(name="Dr. Weird", secret_name="Steve Weird", age=36)
        hero_cap = Hero(
            name="Captain North America", secret_name="Esteban Rogelios", age=93
        )

        team_preventers.heroes.append(hero_tarantula)
        team_preventers.heroes.append(hero_dr_weird)
        team_preventers.heroes.append(hero_cap)
        session.add(team_preventers)
        session.commit()
        session.refresh(hero_tarantula)
        session.refresh(hero_dr_weird)
        session.refresh(hero_cap)
        print("Preventers new hero:", hero_tarantula)
        print("Preventers new hero:", hero_dr_weird)
        print("Preventers new hero:", hero_cap)


def select_heroes():
    with Session(engine) as session:
        statement = select(Team).where(Team.name == "Preventers")
        result = session.exec(statement)
        team_preventers = result.one()

        print("Preventers heroes:", team_preventers.heroes)


def update_heroes():
    with Session(engine) as session:
        statement = select(Hero).where(Hero.name == "Spider-Boy")
        result = session.exec(statement)
        hero_spider_boy = result.one()

        hero_spider_boy.team = None
        session.add(hero_spider_boy)
        session.commit()

        session.refresh(hero_spider_boy)
        print("Spider-Boy without team:", hero_spider_boy)


def main():
    create_db_and_tables()
    create_heroes()
    select_heroes()
    update_heroes()


if __name__ == "__main__":
    main()

Recap

This chapter was too short for a recap, wasn't it? ๐Ÿค”

Anyway, relationship attributes make it easy and intuitive to work with relationships stored in the database. ๐ŸŽ‰