One-to-one relationship with lift Mapper

Question

How do I implement a One-to-One relationship with Mapper? From Lift wiki:

If you’re looking to model a one-to-one relationship, just use a one-to-many relationship. The only potential hassle is that you’ll have a List[B] instead of a Box[B].

Isn't there a more idiomatic way?

Answer 1

I might approach it by reducing the visibility of the one-to-many relationship and create a getter/setter as such:

protected object comments extends 
  MappedOneToMany(Comment, Comment.post, OrderBy(Comment.id, Ascending))

def comment : Option[Comment] = {
  comments match {
    case Nil => None
    case head :: _ => Some(head)
  }
}

def comment_=(comment: Comment) = {
  comments.clear
  comments += comment
}

Ideal? No. But more idiomatic than having client code deal with the List[Comment] instead of Option[Comment]? I think so.

Along this same line, you could also create your own class which extends MappedOneToMany. This code is untested, but I believe in the spirit of it:

class MappedOneToOne[K,T<:KeyedMapper[K, T],O <: Mapper[O]]
  (meta: MetaMapper[O], foreign: MappedForeignKey[K,O,T], qp: QueryParam[O]*) extends MappedOneToMany(meta, foreign, qp) {

  def get : Option[O] = {
    all match {
      case Nil => None
      case head :: _ => Some(head)
    }
  }

  def set(o: O) : O = {
    clear
    this += o
    o
  }
}

class Foo extends KeyedMapper[Int,Foo] {
  object bar extends MappedOneToOne[Int,Foo,Bar]
}

f.bar.get match {
  case Some(bar) => println("Got bar")
  case _ =>
}