Sangria Scalar for ISO 8601 Dates

08 May 2020

Today I had to figure out how to encode a java.util.Date as a GraphQL Scalar using Sangria. Scalars in GraphQL are basically fancy strings that let you encode custom data types. This one reads and writes a java.util.Date as an ISO 8601-formatted string, which looks like “2020-05-09T03:20:28Z.” If your dates are not in UTC you may have a bad time, so don’t do that.

Here it is!

import scala.util.Try
import java.time.format.DateTimeFormatter
import java.time.Instant
import java.util.{Date, TimeZone}
import sangria.validation.ValueCoercionViolation
import sangria.schema._
import sangria.ast._

// Helper object for doing the decoding. I put this in a utils package
object ISO8601 {
  val tz = TimeZone.getTimeZone("UTC").toZoneId
  val formatter = DateTimeFormatter.ISO_INSTANT.withZone(tz)
  def parse(s: String): Try[Date] = Try {
    val temporalAccessor  = formatter.parse(s)
    val instant = Instant.from(temporalAccessor)
    Date.from(instant)
  }

  def encode(date: Date): String = {
    val localDateTime = date.toInstant.atZone(tz).toLocalDateTime
    formatter.format(localDateTime)
  }
}

object Scalars {
  private def parseDate(dt: String): Either[ValueCoercionViolation, Date] = {
    ISO8601.parse(dt).toOption.toRight(InvalidDateTimeViolation)
  }

  case object InvalidDateTimeViolation
    extends ValueCoercionViolation("Input is not valid Date.")
  // The actual Scalar definition. Import this implicit when defining your schema
  // to be enable Sangria to infer schemas for case classes that have Date memebers
  implicit val DateType = ScalarType[Date](
    "Date",
    coerceOutput = (date, _) => ISO8601.encode(date),
    coerceInput = {
      case StringValue(dt, _, _ , _, _) => parseDate(dt)
      case _ => Left(InvalidDateTimeViolation)
    },
    coerceUserInput = {
      case s: String => parseDate(s)
      case _ => Left(InvalidDateTimeViolation)
    }
  )
}