How to serialize/deserialize dynamic field names using Play's json - scala

Question

I'm using Play framework 2.2.2.
I'm trying to handle json request like this one
[
{
"id" : "123",
"language" : "en",
"text" : "This is an example of a text",
"Metadata_IP" : "192.168.20.34",
"Metadata_date" : "2001-07-04T12:08:56.235-0700"
},
{
"id" : "124",
"language" : "en",
"text" : "Some more text here",
"Metadata_IP" : "192.168.20.31",
"Metadata_date" : "2001-07-04T12:09:56.235-0700",
"Metadata_name" : "someone"
}
]
The Metadata_ field are dynamic field meaning the user can send what ever he want (eg. Metadata_color, etc...)
What is the best way to handle this?
Can I use Readers with deserialize it to case class? How can I do this? I guess the dynamic field will be Map[String, String], but how should I make the reader parse this?
Thanks

Answer 1

Something like this could work:
implicit object jsObjToKeyValueSeq extends Reads[Seq[(String, String)]] {
override def reads(json: JsValue) = json match {
case js: JsObject =>
JsSuccess(js.fields.collect { case (key, JsString(value)) => key -> value })
case x => JsError(s"Unexpected json: $x")
}
}

Answer 2

We have faced the exact problem and solved it using a custom implementation. The solution is detailed here
Example:
Scala class
case class Person(name: String, age: String, customFields: Map[String,String])
Default Json representation of above class will be:
{
"name": "anil",
"age": "30",
"customFields": {
"field1": "value1",
"field2": "value2"
}
}
But what we wanted was:
{
"name": "anil",
"age": "30",
"field1": "value1",
"field2": "value2"
}
This was not very straight forward. While this could be possible using play framework, we didn’t want to complicate things too much. Finally we found a way to do it by returning a Map[String, String] which represents each class (it’s fields & values) using reflection and handle the behavior for custom fields separately.
case class Person(name: String, age: String, customFields:CustomFields)
case class CustomFields(valueMap: Map[String,String])
def covertToMap(ref: AnyRef) =
ref.getClass.getDeclaredFields.foldLeft(Map[String, Any]()){
(map, field) => {
field.setAccessible(true)
val value = field.get(ref)
value match {
case c: CustomFields => {
map ++ c.valueMap
}
case _ => {
map + (field.getName -> value)
}
}
}
}
Use the covertToMap() to convert any case class to a Map and then convert this map to normal Json using jackson json4s.
val json = Serialization.write(covertToMap(person))
Complete source code is available here