Json4s: deserializing specific fields using a custom serializer

Question

I have a case class with many members, two of which are not primitive:

import com.twitter.util.Duration case class Foo( a: Int, b: Int, ..., y: Int, z: Int, timeoutSeconds: Duration, runtimeMinutes: Duration) 

I would like to deserialize the following JSON into an instance of this case class:

 { "a": 1, "b": 2, // ... "y": 42, "z": 43, "timeoutSeconds": 30, "runtimeMinutes": 12, } 

Usually I just write json.extract[Foo] . However, I get an obvious MappingException with this due to timeoutSeconds and runtimeMinutes .

I looked at the FieldSerializer , which allows you to convert fields to AST. However, this is not enough because it allows only AST transformations.

I also looked at the CustomSerializer[Duration] extension, but there is no way to figure out which JSON keys are processed ( timeoutSeconds or runtimeMinutes ).

I could also try extending CustomSerializer[Foo] , but then I will have many code templates to extract the values ​​for a , b , ..., z .

Ideally, I need something that accepts PartialFunction[JField, T] as a deserializer, so that I can simply write:

 { case ("timeoutSeconds", JInt(timeout) => timeout.seconds case ("runtimeMinutes", JInt(runtime) => runtime.minutes } 

and rely on case class deserialization for the rest of the parameters. Is such a design possible in json4s?

Note that this is similar to a combination of type and field serializers , except that I want type deserialization to differ depending on the JSON key.