Field 'age2' can only be read from 'application/x-www-form-urlencoded' form content

Question

Could you guys tell me why following form of extraction works for both multipart/form-data and x-www-form-urlencoded requests

formFields("firstName"?, "age2"?, "sex", "vip"?) {
  (firstName : Option[String], age2, sex, vip) =>
    complete(firstName + "." + age2 + "." + sex + "." + vip)
}

but this, seemingly equal, version of code

formFields('firstName :: ("age2".as[Int]) :: ('sex?) :: ("VIP" ? false) :: HNil) {
  (firstName : String, age2 : Int, sex : Option[String], vip : Boolean) =>
    complete(firstName + "." + age2 + "." + sex + "." + vip)
}

doesn't work when request is multipart/form-data encoded.

Server returns following error:

There was a problem with the requests Content-Type:
Field 'age2' can only be read from 'application/x-www-form-urlencoded' form content

I see a call-stack to spray.httpx.unmarshalling.FormField.fail but my Scala skills are not strong enough to decipher it.

Answer 1

"Implementing a default unmarshaler for int" can be done like this:

object JsonImplicits extends DefaultJsonProtocol {
    implicit val IntUnmarshaller = Unmarshaller.delegate[String, Int](MediaRanges.`*/*`) { _.toInt }
}

And making sure its in scope before your route definition by adding the import right before: import JsonImplicits._

Answer 2

Turned out there is no default unmarshaler for Int. So if you implement one it all works.

Not sure why it wasn't done by maintainers in the first place.