title: Play JSON formatter for Map[Int,_]
body: <p>I am attempting to migrate a Rails/Mongodb application to Play 2.3 using play-reactivemongo and reactivemongo-extensions. In modeling my data I am running across a problem serializing and deserializing a Map[Int,Boolean].</p>
  
  <p>When I try to define my formats via macro like so</p>
  
  <pre><code>implicit val myCaseClass = Json.format[MyCaseClass]
  </code></pre>
  
  <p>where MyCaseClass has a few string fields, a BSONObjectID field, and a Map[Int,Boolean] field the compiler complains with:</p>
  
  <pre><code>No Json serializer found for type Map[Int,Boolean]. Try to implement an implicit Writes or Format for this type.
  No Json deserializer found for type Map[Int,Boolean]. Try to implement an implicit Reads or Format for this type.
  </code></pre>
  
  <p>Looking at the source code for Play in Reads.scala I see a Reads defined for Map[String,_] but none for Map[Int,_]. </p>
  
  <p>Is there a reason why Play has default Read/Writes for string maps but not for other simple types? </p>
  
  <p>I don't fully understand the Map[String,_] defined by play because I am fairly new to scala. How would I go about translating that into a Map[Int,_]? If that is not possible for some technical reason how would I define a Reads/Writes for Map[Int,Boolean]?</p>