Scala: Copying case classes with trait - scala

Question

I'm fairly new to Scala and I have a question about the best way to copy a case class while preserving data that comes from traits. For example, let's say I have the following:
trait Auditing {
var createTime: Timestamp = new Timestamp(System.currentTimeMillis)
}
case class User(val userName: String, val email: String) extends Auditing
val user = User("Joe", "joe#blah.com")
Then I want to make a new copy with one parameter changed:
val user2 = user.copy(email = "joe#newemail.com")
Now, in the example above, the property createTime does not get copied over because it is not defined in the constructor of the User case class. So my question is: assuming that moving createTime into the constructor is not an option, what is the best way for getting a copy of the User object that includes the value from the trait?
I'm using Scala 2.9.1
Thanks in advance!
Joe

Answer 1

You can override the copy method with that behavior.
case class User(val userName: String, val email: String) extends Auditing
{
def copy(userName = this.userName, email = this.email) {
val copiedUser = User(userName, email)
copiedUser.createTime = createTime
copiedUser
}
}

Answer 2

While I see no other solution than Reuben's, I don't understand the requirement to leave the constructor args untouched. This would be the most natural solution:
case class User(userName: String, email: String,
override val createTime:Timestamp = new Timestamp(System.currentTimeMillis))
extends Auditing
If you don't want the user to be able to overwrite createTime, you can still use:
case class User private (userName: String, email: String,
override val createTime:Timestamp) extends Auditing {
def this(userName: String, email: String) =
this(userName, email, new Timestamp(System.currentTimeMillis))
}
The only drawback is that you need to write new User("Joe", "joe#blah.com"), as the primary constructor is now private.

Answer 3

You might be better of not using a case class. You can easily implement the
functionality you need yourself. The below code implements the copy method you wanted, a constructor without new, hides the original constructor, and creates an extractor so that you can use User in case statements.
class User private(val userName: String,
val email: String,
val timeStamp: Timestamp =
new Timestamp(System.currentTimeMillis)) {
def copy(uName: String = userName,
eMail: String = email) =
new User(uName, eMail, timeStamp)
}
object User {
def apply(userName: String, email: String) =
new User(userName, email)
def unapply(u: User) = Some((u.userName, u.email, u.timeStamp))
}