Model relationships and enforce database constraints

In this lesson you will model relationships and enforce database constraints. Work with the Equipment Rental API in the downloadable lab. The reference is a complete solution with separate lesson checks, so you can inspect the answer, make a deliberate local change and verify its behavior.

A rental belongs to one equipment item and one user. Foreign keys express those relationships in the database, so a row cannot reference an owner that does not exist. The Equipment.rentals relationship lets Python navigate the association; it does not replace the foreign key. A unique equipment name and check constraints for positive quantities protect writes from any client, including scripts that bypass the API model.

Choose constraints from a concrete invariant. A quantity of zero cannot reserve equipment, and a daily rate cannot be negative. The database rejects either even if application validation is accidentally skipped. Status has a closed set of values, rather than accepting arbitrary strings. These row constraints cannot enforce total active rentals against equipment capacity by themselves: that rule spans rows and needs a transaction protocol in lesson 10. The relationships use lazy='raise' so accidental attribute access cannot quietly issue more queries. Explicit loading is introduced later. Do not solve referential errors by disabling constraints; fix the ordering or validity of the write.

Worked source: equipment/models.py, Rental.

Compare User, Equipment and Rental in models.py. The lesson06 check attempts a rental for a nonexistent UUID owner and separately inserts equipment with zero capacity. Both writes must fail at the database boundary and roll back. Check that no row from either failed transaction remains.

class Rental(Base):
    __tablename__ = "rentals"
    id: Mapped[uuid.UUID] = mapped_column(Uuid, primary_key=True, default=uuid.uuid4)
    equipment_id: Mapped[int] = mapped_column(ForeignKey("equipment.id"))
    owner_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"))
    quantity: Mapped[int]
    status: Mapped[str] = mapped_column(String(20), default="active")
    created_at: Mapped[datetime] = mapped_column(
        DateTime(timezone=True), server_default=func.now()
    )
    equipment: Mapped[Equipment] = relationship(back_populates="rentals", lazy="raise")
    __table_args__ = (
        CheckConstraint("quantity >= 1", name="positive_rental_quantity"),
        CheckConstraint("status IN ('active','checked_in')", name="rental_status"),
    )

python run_checks.py -k lesson06