Question

I’m using Salat with MongoDB and I’m trying to convert to natural keys to avoid duplicates in the database. The case class I’m using looks somewhat like:

case class Foo(someRelatedId: String, email: String ...)

I would like to add a natural key that consists of someRelatedId+email and have MongoDB use that instead of the default ObjectId. From the documentation I get the feeling it is possible, but I’m still groping around for a working solution. This is in a large part due to my lack of proficiency with Scala itself, I’m sure.

Update: I have a working solution now, but I’m still wondering if it is the best way to go

case class Foo(someRelatedId: String, email: String, naturalKey: String)

object Foo {
  def apply((someRelatedId: String, email: String) {
    apply(someRelatedId, email, someRelatedId+email)
  }
}

And then in package.scala I map to a custom salat context:

implicit val ctx = new Context() {
  val name = Some("Custom Context")
}
ctx.registerGlobalKeyOverride(remapThis = "naturalKey", toThisInstead = "_id")

This way I avoid having a mandatory (meaningless) _id field in my domain classes, but I do have to overload apply() on the companion object, which seems a bit clunky.

Answer 1

main Salat developer here.

Like Milan suggested, create a case class for your composite key:

case class FooKey(someRelatedId: String, email: String)

case class Foo(@Key("_id") naturalKey: FooKey) {

  // use @Persist if you want these fields serialized verbatim to Mongo - see https://github.com/novus/salat/wiki/Annotations for details
  @Persist val email =  naturalKey.email
  @Persist val someRelatedId = naturalKey.someRelatedId

}

object FooDAO extends SalatDAO[Foo, FooKey](collection = /*  some Mongo coll */ )

If you object to “_id” as a field name, you can use a global override in the context to remap “_id” to “naturalKey”, or supply ad hoc @Key overrides on each object.

I don’t personally like giving the _id a different name in your models as then your Mongo queries must use the serialized key “_id” while all your business logic must use the case class field name (“naturalKey” or whatever), but YMMV.

P.S. Our mailing list is at http://groups.google.com/group/scala-salat – I’ll see your question quicker there than Stack Overflow.