Play 2.4 Json Formats with Recursive Types

Play24JsonFormatsWithRecursiveTypes.scala

import play.api.libs.functional.syntax._
import play.api.libs.json._

case class Foo(id: String, bar: Option[Bar])
object Foo {
  implicit val format = Json.format[Foo]
}

case class Bar(id: String, foos: Option[Seq[Foo]])
object Bar {
  implicit val format: Format[Bar] = (
    (__ \ 'id).format[String] and
    (__ \ 'foos).lazyFormatNullable(implicitly[Format[Seq[Foo]]])
  )(Bar.apply, unlift(Bar.unapply))
}

// foo1 contains bar1 which contains foo2
val jsonStr = """{ "id": "foo1", "bar": { "id": "bar1", "foos": [ { "id": "foo2" } ] } }"""
val json = Json.parse(jsonStr)
val foo = json.as[Foo]

// Did it work?
foo.bar.flatMap(_.foos).flatMap(_.headOption).map(_.id).contains("foo2")

// Complete round trip
Json.toJson(foo)

Same in json4s: https://gist.github.com/lasiltan/b8bd06c292b42245f57e