mappers.py

mapper_reg.map_imperatively(
    AlbumImage,
    album_images_t,
    properties={
        "album": relationship(
            Album, backref=backref("images", order_by=album_images_t.c.display_order)
        ),
        "image": relationship(Image, backref="album_images"),
    },
)

snippet.py

                ai = AlbumImage(
                    #album=album,
                    display_order=index,
                    image=album_image_dict["image"],
                    caption=album_image_dict["caption"],
                )
                album.images.append(ai)