model CD {
   has           $!artist-id is referencing{ Artist.id };
   has Artist $.author is relationship{ .artist-id };
}
model Artist {
   has UInt $.id is column{ :id };
   has CD   @.cds is relationship{ .artist-id };
}

The trait will receive a block and run it passing: if the type of the attribute is positional the attribute type, else the model’s type. It’s return should be a column that is referencing some other column. It will create a new result seq using that.