ScalaModel.scala

class ScalaModel[T] {
  
  def valid_? = {
    errors.clear
    validate
    errors.empty_?
  }
  
  def validate {}
  
  def req(field: String, fun: => Boolean, msg: String){
    if(!fun) errors.add(field, msg)
  }
  
  object errors {
    val fields = Map[String, ListBuffer[String]]()
    
    def clear = fields.clear
    
    def empty_? = fields.isEmpty
    
    def add(field: String, msg: String){
      if(!fields.contains(field)){
        fields(field) = new ListBuffer[String]
      }
      fields(field) += msg
    }
    
    def full = fields.values.toList.flatten
  } 
}

test.scala

scala> val u = new User
u: models.User = models.User@9c98e8

scala> u.valid_?
res12: Boolean = false

scala> u.errors.full
res14: List[String] = List(Underage!, Username is too short)

scala> u.username = "foobarbaz"

scala> u.valid_?
res15: Boolean = false

scala> u.errors.full
res16: List[String] = List(Underage!)

scala> u.age = 30

scala> u.valid_?
res17: Boolean = true

scala> u.errors.full
res18: List[String] = List()

User.scala

class User extends ScalaModel {
  var username: String = ""
  var age: Int = 0
  
  override def validate {
    req("username", username.length > 6, "Username is too short")
    req("age", age > 18, "Underage!")
  }
}