I have a users table and a roles table, such that a User optionally has a single Role:

class User(Base):
    id: Mapped[int] = mapped_column(primary_key=True)

    role: Mapped[Optional["Role"]] = relationship(back_populates="user")
    # ...


class Role(Base):
    id: Mapped[int] = mapped_column(primary_key=True)
    user_id: Mapped[int] = mapped_column(ForeignKey("user.id"))

    user: Mapped[Optional["User"]] = relationship(back_populates="role")
    # ...

Now in code I would like that User.role always return a Role. If a specific User doesn’t have a Role, it should just return a new default one: Role().

How can I replace User.role with some kind of getter function that does this?

A somewhat obvious solution would be:

class User(Base):
    # ...
    @property
    def role_or_new():
        return self.role or Role()

However, I kind of want to override the default attribute name, role.

There are some similarly named existing questions, but I think they’re all subtly different:

• Flask-SQLAlchemy set relationship default value: solved through events
• How to set a default value for a relationship in SQLAlchemy?: Adds a new method, but I want to modify how the existing attribute role behaves
• How to set a default value from a SQLAlchemy relationship?: Asks how to get default values based of an existing relationship