my_command.scala

class MyAppBinding[T](b: FieldDescriptor[T]) {
  def unique[TId](collection: MongoCollection, currentItem: Option[TId] = None) = b.validateWith(_ ⇒ {
    _ flatMap { Validators.uniqueField(b.name, collection, currentItem).validate(_) }
  })
}

abstract class MyAppCommand[S](implicit mf: Manifest[S], protected val jsonFormats: Formats) extends ModelCommand[S] with JsonCommand {
  type Result = S
    
  implicit def myAppValidators[T](b: FieldDescriptor[T]) = new MyAppBinding[T](b)

}

unique_field_mongo.scala

object Validators {
  def uniqueField[TId, TResult](fieldName: String, collection: MongoCollection, currentItem: Option[TId] = None): Validator[TResult] = {
    new Validator[TResult] {
      def q(s: TResult) = Map(fieldName -> s)
      def newQ(s: TResult) = currentItem.fold(id ⇒ q(s) ++ Map("_id" -> ("$ne" -> id)), q(s))
      def validate[R >: TResult <: TResult](subject: R): FieldValidation[R] = {
        if (collection.count(newQ(subject), Map(fieldName -> 1)) == 0) subject.success
        else ValidationError(fieldName.humanize + " exists already.", FieldName(fieldName), NotUnique).fail
      }
    }
  }
}

object Validations {

  def uniqueField[TId, TResult](fieldName: String, value: ⇒ TResult, collection: MongoCollection, currentItem: Option[TId] = None): FieldValidation[TResult] = {
    Validators.uniqueField(fieldName, collection, currentItem).validate(value)
  }
}