Add KeyableFormat for Map json serialization (#125).

src/main/scala/spray/json/CollectionFormats.scala

@@ -44,10 +44,9 @@ trait CollectionFormats {
   }
 
   /**
-    * Supplies the JsonFormat for Maps. The implicitly available JsonFormat for the key type K must
-    * always write JsStrings, otherwise a [[spray.json.SerializationException]] will be thrown.
+   * Supplies the JsonFormat for Maps.
    */
-  implicit def mapFormat[K :JsonFormat, V :JsonFormat] = new RootJsonFormat[Map[K, V]] {
+  def mapFormat[K :JsonFormat, V :JsonFormat] = new RootJsonFormat[Map[K, V]] {
     def write(m: Map[K, V]) = JsObject {
       m.map { field =>
         field._1.toJson match {
@@ -64,6 +63,23 @@ trait CollectionFormats {
     }
   }
 
+  /**
+   * Supplies the JsonFormat for Maps.
+   */
+  implicit def keyableMapFormat[K :KeyableFormat, V :JsonFormat] = new RootJsonFormat[Map[K, V]] {
+    def write(m: Map[K, V]) = JsObject {
+      m.map { field =>
+        field._1.toKey -> field._2.toJson
+      }
+    }
+    def read(value: JsValue) = value match {
+      case x: JsObject => x.fields.map { field =>
+        (JsString(field._1).fromKey, field._2.convertTo[V])
+      }
+      case x => deserializationError("Expected Map as JsObject, but got " + x)
+    }
+  }
+
   import collection.{immutable => imm}
 
   implicit def immIterableFormat[T :JsonFormat]   = viaSeq[imm.Iterable[T], T](seq => imm.Iterable(seq :_*))

src/main/scala/spray/json/DefaultJsonProtocol.scala

@@ -26,5 +26,6 @@ trait DefaultJsonProtocol
         with CollectionFormats
         with ProductFormats
         with AdditionalFormats
+        with KeyableFormats
 
 object DefaultJsonProtocol extends DefaultJsonProtocol

src/main/scala/spray/json/KeyableFormat.scala

@@ -0,0 +1,11 @@
+package spray.json
+
+trait KeyableWriter[T] {
+  def write(obj: T): String
+}
+
+trait KeyableReader[T] {
+  def read(jsString: JsString): T
+}
+
+trait KeyableFormat[T] extends KeyableWriter[T] with KeyableReader[T]

src/main/scala/spray/json/KeyableFormats.scala

@@ -0,0 +1,9 @@
+package spray.json
+
+trait KeyableFormats {
+
+  implicit object StringKeyableFormat extends KeyableFormat[String] {
+    override def write(obj: String) = obj
+    override def read(json: JsString) = json.value
+  }
+}

src/main/scala/spray/json/package.scala

@@ -28,6 +28,7 @@ package object json {
 
   implicit def enrichAny[T](any: T) = new RichAny(any)
   implicit def enrichString(string: String) = new RichString(string)
+  implicit def enrichJsString[T](jsString: JsString) = new RichJsString(jsString)
 
   @deprecated("use enrichAny", "1.3.4")
   def pimpAny[T](any: T) = new PimpedAny(any)
@@ -42,6 +43,7 @@ package json {
 
   private[json] class RichAny[T](any: T) {
     def toJson(implicit writer: JsonWriter[T]): JsValue = writer.write(any)
+    def toKey(implicit writer: KeyableWriter[T]): String = writer.write(any)
   }
 
   private[json] class RichString(string: String) {
@@ -51,6 +53,10 @@ package json {
     def parseJson(settings: JsonParserSettings): JsValue = JsonParser(string, settings)
   }
 
+  private[json] class RichJsString(jsString: JsString) {
+    def fromKey[T](implicit reader: KeyableReader[T]): T = reader.read(jsString)
+  }
+
   @deprecated("use RichAny", "1.3.4")
   private[json] class PimpedAny[T](any: T) {
     def toJson(implicit writer: JsonWriter[T]): JsValue = writer.write(any)

src/test/scala/spray/json/CollectionFormatsSpec.scala

@@ -52,8 +52,27 @@ class CollectionFormatsSpec extends Specification with DefaultJsonProtocol {
     "be able to convert a JsObject to a Map[String, Long]" in {
       json.convertTo[Map[String, Long]] mustEqual map
     }
-    "throw an Exception when trying to serialize a map whose key are not serialized to JsStrings" in {
-      Map(1 -> "a").toJson must throwA(new SerializationException("Map key must be formatted as JsString, not '1'"))
+    "convert a Map[Int, String] to a JsObject given a KeyableFormat" in {
+      implicit val keyableFormat: KeyableFormat[Int] = new KeyableFormat[Int] {
+        override def read(jsString: JsString) = jsString.value.toInt
+        override def write(obj: Int) = obj.toString
+      }
+
+      val map = Map(1 -> "a", 2 -> "b", 3 -> "c")
+      val json = JsObject("1" -> JsString("a"), "2" -> JsString("b"), "3" -> JsString("c"))
+
+      map.toJson mustEqual json
+    }
+    "be able to convert a JsObject to a Map[Int, String] given a KeyableFormat" in {
+      implicit val keyableFormat: KeyableFormat[Int] = new KeyableFormat[Int] {
+        override def read(jsString: JsString) = jsString.value.toInt
+        override def write(obj: Int) = obj.toString
+      }
+
+      val map = Map(1 -> "a", 2 -> "b", 3 -> "c")
+      val json = JsObject("1" -> JsString("a"), "2" -> JsString("b"), "3" -> JsString("c"))
+
+      json.convertTo[Map[Int, String]] mustEqual map
     }
   }