Special ForeignModel and BackRef for one-to-one relations

[gammelalf]

#[derive(Model)]
struct Data {
    #[rorm(id)]
    id: i64,
    
    extras: BackRef<field!(Extra::F.data)>,
}

#[derive(Model)]
struct Extra {
    #[rorm(id)]
    id: i64,
    
    data: ForeignModel<Data>,
}

This is a classic many-to-one relation.

By adding #[rorm(unique)] to data: ForeignModel<Data>, you turn it into a one-to-one relation.
(technically "one-to-optional" would be more fitting, because a Data could still have zero Extra, but not more than one).

However the BackRef on Data doesn't take not of this change. I'd like some specialized version of BackRef which only connects to #[rorm(unique)] ForeignModels to provide extra semantics and methods.