Considering the following MRE:

from sqlalchemy import ForeignKey
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import relationship

class Base(DeclarativeBase):
    pass

class Item(Base):
    __tablename__ = "item"
    id: Mapped[int] = mapped_column(primary_key=True)
    box_id: Mapped["Box"] = mapped_column(ForeignKey("box.id"))
    owner_id: Mapped["Owner"] = mapped_column(ForeignKey("owner.id"))
    box: Mapped["Box"] = relationship(back_populates="items")
    owner: Mapped["Owner"] = relationship(back_populates="items")

class Box(Base):
    __tablename__ = "box"
    id: Mapped[int] = mapped_column(primary_key=True)
    items: Mapped[list["Item"]] = relationship()

class Owner(Base):
    __tablename__ = "owner"
    id: Mapped[int] = mapped_column(primary_key=True)
    items: Mapped[list["Item"]] = relationship()
    # association proxy/hybrid property here

…at the last line (commented) I’d like some way to access a list of unique Box es for each Owner.

If I add the following association proxy to class Owner:

boxes: AssociationProxy[list["Box"]] = association_proxy("items", "box")

… this partially adresses the goal, though the list is not unique len(boxes) == len(items).

To only return unique Box es, I can add the following hybrid property:

@hybrid_property
def unique_boxes(self):
    return list(set(self.boxes))

… or any other “uniquifying” function run on the association proxy boxes.

Is there a more graceful way to achieve this outcome? Perhaps one which doesn’t require the intermediary association proxy?