Reader and writer in order to work with spray-json and reactivemongo. Based on https://github.com/zenexity/Play-ReactiveMongo.

JsBSONHandlers.scala

import spray.json._
import reactivemongo.bson._
import reactivemongo.bson.handlers.{ BSONReader, BSONWriter, RawBSONWriter }
import scala.util.{ Try, Success, Failure }
import org.apache.commons.codec.binary.Hex
import org.joda.time.format.ISODateTimeFormat
import org.joda.time.{ DateTime, DateTimeZone }
import java.nio.ByteBuffer
import org.jboss.netty.buffer.ChannelBuffers

/** Readers and Writers for Reactivemongo BSON and Spray Json. 
  *
  * Credits: 
  * It is based on the work Stephane Godbillon and Pascal Voitot on play reactivemongo plugin
  * @see https://github.com/zenexity/Play-ReactiveMongo
  */
trait JsHandling {

  //** Helper which allow to get json values from reactivemongo. */
  implicit object JsObjectReader extends BSONReader[JsObject] {
    def fromBSON(doc: BSONDocument): JsObject = JsBSONReader.readObject(doc.toTraversable)
  }

  /** Helper which allow to send json values to reactivemongo. */
  object JsObjectWriter extends BSONWriter[JsObject] {
    def toBSON(doc: JsObject) = JsBSONWriter.writeObject(doc)
  }

  /** Helper which allow to send json values to reactivemongo. */
  implicit object JsValueWriter extends RawBSONWriter[JsValue] {
    def write(doc: JsValue) = {
      doc match {
        case o: JsObject => JsBSONWriter.writeObject(o).makeBuffer
        case a: JsArray  => JsBSONWriter.writeArray(a).makeBuffer
        case _           => throw new RuntimeException("JsValue can only be JsObject/JsArray")
      }
    }
  }
}

/** Readers
  *
  * @see http://docs.mongodb.org/manual/reference/mongodb-extended-json/
  */
object JsBSONReader {

  def readObject(doc: TraversableBSONDocument) = JsObject(doc.iterator.toSeq.map(readElement): _*)

  def readElement(e: BSONElement): (String, JsValue) = e.name -> (e.value match {
    case BSONString(value)            => JsString(value)
    case BSONInteger(value)           => JsNumber(value)
    case BSONLong(value)              => JsNumber(value)
    case BSONDouble(value)            => JsNumber(value)
    case BSONBoolean(true)            => JsTrue
    case BSONBoolean(false)           => JsFalse
    case BSONNull                     => JsNull
    case doc: TraversableBSONDocument => readObject(doc)
    case doc: AppendableBSONDocument  => readObject(doc.toTraversable)
    case arr: TraversableBSONArray    => readArray(arr)
    case arr: AppendableBSONArray     => readArray(arr.toTraversable)
    case oid @ BSONObjectID(value)    => JsObject("$oid" -> JsString(oid.stringify))
    case BSONDateTime(value)          => JsString(isoFormatter.print(value)) // Doesn't follow mongdb-extended
    case bb: BSONBinary               => readBSONBinary(bb)
    case BSONRegex(value, flags)      => JsObject("$regex" -> JsString(value), "$options" -> JsString(flags))
    case BSONTimestamp(value) => JsObject("$timestamp" -> JsObject(
      "t" -> JsNumber(value.toInt), "i" -> JsNumber((value >>> 32).toInt)))
    case BSONUndefined           => JsObject("$undefined" -> JsTrue)
    // case BSONMinKey                   => JsObject("$minKey" -> JsNumber(1)) // Bug on reactivemongo
    case BSONMaxKey              => JsObject("$maxKey" -> JsNumber(1))
    // case BSONDBPointer(value, id) => JsObject("$ref" -> JsString(value), "$id" -> JsString(Hex.encodeHexString(id))) // Not implemented
    // NOT STANDARD AT ALL WITH JSON and MONGO
    case BSONJavaScript(value)   => JsObject("$js" -> JsString(value))
    case BSONSymbol(value)       => JsObject("$sym" -> JsString(value))
    case BSONJavaScriptWS(value) => JsObject("$jsws" -> JsString(value))
  })

  def readArray(array: TraversableBSONArray) = JsArray(array.iterator.toSeq.map(readElement(_)._2): _*)

  def readBSONBinary(bb: BSONBinary) = {
    val arr = new Array[Byte](bb.value.readableBytes())
    bb.value.readBytes(arr)
    val sub = ByteBuffer.allocate(4).putInt(bb.subtype.value).array
    JsObject("$binary" -> JsString(Hex.encodeHexString(arr)),
      "$type" -> JsString(Hex.encodeHexString(sub)))
  }

  val isoFormatter = ISODateTimeFormat.dateTime.withZone(DateTimeZone.UTC)
}

/** Writers
  *
  * @see http://docs.mongodb.org/manual/reference/mongodb-extended-json/
  */
object JsBSONWriter {

  def writeObject(obj: JsObject): BSONDocument = BSONDocument(obj.fields.map(writePair).toSeq: _*)

  def writeArray(arr: JsArray): BSONArray =
    BSONArray(arr.elements.zipWithIndex.map(p => writePair(p._2.toString, p._1)).map(_._2): _*)

  def writePair(p: (String, JsValue)): (String, BSONValue) = (p._1, p._2 match {
    case JsString(str @ IsoDateTime(y, m, d, h, mi, s, ms)) => manageDate(y, m, d, h, mi, s, ms) match {
      case Success(dt) => dt
      case Failure(_)  => BSONString(str)
    }
    case JsString(str)   => BSONString(str)
    case JsNumber(value) => BSONDouble(value.doubleValue)
    case obj: JsObject   => manageSpecials(obj)
    case arr: JsArray    => writeArray(arr)
    case JsTrue          => BSONBoolean(true)
    case JsFalse         => BSONBoolean(false)
    case JsNull          => BSONNull
  })

  def manageDate(year: String, month: String, day: String, hour: String, minute: String, second: String, milli: String) =
    Try(BSONDateTime((new DateTime(year.toInt, month.toInt, day.toInt, hour.toInt,
      minute.toInt, second.toInt, milli.toInt, DateTimeZone.UTC)).getMillis))

  def manageSpecials(obj: JsObject): BSONValue =
    if (obj.fields.size > 2) writeObject(obj)
    else (obj.fields.toList match {
      case ("$oid", JsString(str)) :: Nil            => Try(BSONObjectID(Hex.decodeHex(str.toArray)))
      case ("$undefined", JsTrue) :: Nil             => Success(BSONUndefined)
      // case ("$minKey", JsNumber(n)) :: Nil if n == 1 => Success(BSONMinKey) // Bug on reactivemongo
      case ("$maxKey", JsNumber(n)) :: Nil if n == 1 => Success(BSONMaxKey)
      case ("$js", JsString(str)) :: Nil             => Success(BSONJavaScript(str))
      case ("$sym", JsString(str)) :: Nil            => Success(BSONSymbol(str))
      case ("$jsws", JsString(str)) :: Nil           => Success(BSONJavaScriptWS(str))
      case ("$timestamp", ts: JsObject) :: Nil       => manageTimestamp(ts)
      case ("$regex", JsString(r)) :: ("$options", JsString(o)) :: Nil =>
        Success(BSONRegex(r, o))
      case ("$binary", JsString(d)) :: ("$type", JsString(t)) :: Nil =>
        Try(BSONBinary(ChannelBuffers.wrappedBuffer(Hex.decodeHex(d.toArray)),
          findSubtype(Hex.decodeHex(t.toArray))))
      // case ("$ref", JsString(v)) :: ("$id", JsString(i)) :: Nil => // Not implemented
      //   Try(BSONDBPointer(v, Hex.decodeHex(i.toArray)))
      case _ => Success(writeObject(obj))
    }) match {
      case Success(v) => v
      case Failure(_) => writeObject(obj)
    }

  def manageTimestamp(o: JsObject) = o.fields.toList match {
    case ("t", JsNumber(t)) :: ("i", JsNumber(i)) :: Nil =>
      Success(BSONTimestamp((t.toLong & 4294967295L) | (i.toLong << 32)))
    case _ => Failure(new IllegalArgumentException("Illegal timestamp value"))
  }

  def findSubtype(bytes: Array[Byte]) =
    ByteBuffer.wrap(bytes).getInt match {
      case 0x00 => Subtype.GenericBinarySubtype
      case 0x01 => Subtype.FunctionSubtype
      case 0x02 => Subtype.OldBinarySubtype
      case 0x03 => Subtype.UuidSubtype
      case 0x05 => Subtype.Md5Subtype
      // case 0X80 => Subtype.UserDefinedSubtype // Bug on reactivemongo
      case _    => throw new IllegalArgumentException("unsupported binary subtype")
    }

  val IsoDateTime = """^(\d{4,})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})\.(\d{3})Z$""".r
}

I made some changes to make it compile with the latest reactive mongo library: https://gist.github.com/rleibman/7103325d0193be268ed7