Scala Reflection to update a case class val

Question

I\'m using scala and slick here, and I have a baserepository which is responsible for doing the basic crud of my classes. For a design decision, we do have updatedTime and c

Answer 1

As comments have said, don't change a val using reflection. Would you that with a java final variable? It makes your code do really unexpected things. If you need to change the value of a val, don't use a val, use a var.

trait HasCreatedAt {
    var createdAt: Option[DateTime] = None
}

case class User(name:String) extends HasCreatedAt

Although having a var in a case class may bring some unexpected behavior e.g. copy would not work as expected. This may lead to preferring not using a case class for this.

Another approach would be to make the insert method return an updated copy of the case class, e.g.:

trait HasCreatedAt {
    val createdAt: Option[DateTime]
    def withCreatedAt(dt:DateTime):this.type
}

case class User(name:String,createdAt:Option[DateTime] = None) extends HasCreatedAt {
    def withCreatedAt(dt:DateTime) = this.copy(createdAt = Some(dt))
}

trait BaseRepo[ID, R <: HasCreatedAt] {
    def insert(r: R)(implicit session: Session): (ID, R) = {
        val id = ???//insert into db
        (id, r.withCreatedAt(??? /*now*/))
    }
}

EDIT:

Since I didn't answer your original question and you may know what you are doing I am adding a way to do this.

import scala.reflect.runtime.universe._

val user = User("aaa", None)
val m = runtimeMirror(getClass.getClassLoader)
val im = m.reflect(user)
val decl = im.symbol.asType.toType.declaration("createdAt":TermName).asTerm
val fm = im.reflectField(decl)
fm.set(??? /*now*/)

But again, please don't do this. Read this stackoveflow answer to get some insight into what it can cause (vals map to final fields).