Scala Macro for Safe Field Access in MongoDB

ValidAccessor.scala

/**
 * Checks if a field accessor string like "properties.validated" is valid 
 * within nested case classes, e.g.:
 * 
 * case class User(val properties: UserProperties)
 * case class UserProperties(val validated: Boolean)
 * 
 * In the above example, valid[User]("properties.validated") would yield true.
 * 
 * This is useful for compile time checking when doing MongoDB queries 
 * for example. For further details, pitfalls, and why another 
 * solution is better, visit:
 * 
 * http://dominikbucher.wordpress.com/2014/07/07/scala-macros-for-safe-field-access-in-mongodb/
 */
 
import scala.reflect.macros.Context
import scala.language.experimental.macros
import scala.annotation.tailrec

object Accessors {
  def valid[T](accessor: String) = macro valid_impl[T]
  
  def valid_impl[T: c.WeakTypeTag](c: Context)(accessor: c.Expr[String]): c.Expr[String] = {
    import c.universe._
  
    val typeOfT = weakTypeOf[T]
  
    def getFields(tpe: c.universe.Type) = {
      tpe.members.collectFirst {
        case m: MethodSymbol if m.isPrimaryConstructor => m
      }.get.paramss.head
    }
  
    @tailrec
    def hasField(accessor: List[String], tpe: c.universe.Type): Boolean = {
      accessor match {
        case head :: Nil => {
          if (getFields(tpe).filter(_.name.decoded == head).isEmpty) false else true
        }
        case head :: tail => {
          if (getFields(tpe).filter(_.name.decoded == head).isEmpty) false else {
            hasField(tail, tpe.declaration(newTermName(head)).typeSignature)
          }
        }
      }
    }
  
    val accessorRep = accessor.tree match { 
      case Literal(Constant(str: String)) => str 
    }
    val accessorParts = accessorRep.split("\\.").toList
    val validAccessor = hasField(accessorParts, typeOfT)
  
    if (validAccessor) accessor
    else {
      c.error(c.enclosingPosition, s"Field accessor ${accessorRep} of type ${typeOfT} is invalid.")
      accessor
    }
  }
}