Relationships in Python

A foreign key plus relationship gives you objects on both sides.

from sqlalchemy import ForeignKey, create_engine
from sqlalchemy.orm import (DeclarativeBase, Mapped, Session, mapped_column,
                            relationship)


class Base(DeclarativeBase):
    pass


class Tour(Base):
    __tablename__ = "tours"
    id: Mapped[int] = mapped_column(primary_key=True)
    slug: Mapped[str]
    steps: Mapped[list["Step"]] = relationship(back_populates="tour")


class Step(Base):
    __tablename__ = "steps"
    id: Mapped[int] = mapped_column(primary_key=True)
    title: Mapped[str]
    tour_id: Mapped[int] = mapped_column(ForeignKey("tours.id"))
    tour: Mapped[Tour] = relationship(back_populates="steps")


engine = create_engine("sqlite+pysqlite:///:memory:")
Base.metadata.create_all(engine)

with Session(engine) as session:
    tour = Tour(slug="basics")
    tour.steps = [Step(title="Variables"), Step(title="Strings")]
    session.add(tour)
    session.commit()
    print(len(tour.steps), tour.steps[0].tour.slug)

How it works

1. ForeignKey is the column; relationship is the attribute.
2. back_populates keeps the two sides in step.
3. Appending to the collection sets the key for you.

Keywords and builtins used here

• Base
• Step
• Tour
• as
• class
• id
• int
• len
• list
• pass
• print
• str
• with

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Step 5 of 5 in SQLAlchemy, step 17 of 19 in Web services & data access.

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