CHJson.scala

object CHJson extends com.codahale.jerkson.Json {
  // this is where we setup out Jackson module for custom de/serialization
  val module: SimpleModule = new SimpleModule("delving", Version.unknownVersion())
  module.addSerializer(classOf[ObjectId], new ObjectIdSerializer)
  module.addDeserializer(classOf[ObjectId], new ObjectIdDeserializer)
  mapper.registerModule(module)
}

JsonDeSerializers.scala

package extensions

import org.bson.types.ObjectId
import org.codehaus.jackson.{JsonToken, JsonParser, JsonGenerator}
import org.codehaus.jackson.map._
import annotate.JsonCachable

/**
 *
 * @author Manuel Bernhardt <bernhardt.manuel@gmail.com>
 */

@JsonCachable
class ObjectIdSerializer extends JsonSerializer[ObjectId] {
  def serialize(id: ObjectId, json: JsonGenerator, provider: SerializerProvider) {
    json.writeString(id.toString)
  }
}

class ObjectIdDeserializer extends JsonDeserializer[ObjectId] {
  def deserialize(jp: JsonParser, context: DeserializationContext) = {
    if (!ObjectId.isValid(jp.getText)) throw context.mappingException("invalid ObjectId " + jp.getText)
    new ObjectId(jp.getText)
  }
}