harryh

Scala Puzzler of the day

Many different database objects in foursquare have a foreign key to User.  So I wrote something like so:

object User {
  trait FK[T <: FK[T]] extends LongKeyedMapper[T] {
    self: T=>
    object userid extends MappedLongForeignKey(this, User)
    def hasUser_? = userid.defined_?
  }

  class FKList[A <: FK[A]](list: List[A]) {    
    def primeUsers: List[A] = {
      if (!list.isEmpty) {
        val uids = list.filter(_.hasUser_?).map(_.userid.is)
        val users = findAll(ByList(User.id, uids))
        list.foreach(item => {
          item.userid.primeObj(users.find(_.id.is == item.userid.is))
        })
      }
      list
    }
  }
}

So that things can extend User.FK[Foo] and get automatic extra functionality for dealing with their User parents.  But you’ll notice that the field name is hard coded in the above.  Some things have two user parents.  For example Venues have creators and mayors.  Is there some way to enhance the code above so that a single model can extend User.FK twice with different field names?  Pondering what might be a good way to do this.