import org.json4s._
import org.json4s.jackson.JsonMethods._

val a = parse(""" { "id":"6988", "signatories":[ { "fields":[ { "name":"fstname", "value":"Bruce" }, { "name":"sndname", "value":"Lee" }, { "name":"email", "value":"bruce.lee@company.com" }, { "name":"sigco", "value":"Company" }, { "name":"mobile", "value":"0760000000" } ] }, { "fields":[ { "name":"fstname", "value":"" }, { "name":"sndname", "value":"" }, { "name":"email", "value":"" }, { "name":"mobile", "value":"" }, { "name":"sigco", "value":"" } ] } ] } """)

val v = a.\("signatories").\("fields")(1).transform {
   // Each JArray is made of objects. Find fields in the object with key as name and value as fstname
   case obj: JObject => obj.findField(_.equals(JField("name", JString("fstname")))) match {
     case None => obj //Didn't find the field. Return the same object back to the array
     // Found the field. Change the value
     case Some(x) =>
       obj.transformField { case JField(k, v) if k == "value" => JField(k, JString("New name")) }
   }
}

// how do I get the fields[1] into the original JObject "a"
val merged = a.replace(List("fields"), v)

// convert back to json
println(compact(render(merged)))

/*
The result I'm getting
{
    "id": "6988",
    "signatories": [
        {
            "fields": [
                {
                    "name": "fstname",
                    "value": "Bruce"
                },
                {
                    "name": "sndname",
                    "value": "Lee"
                },
                {
                    "name": "email",
                    "value": "[email protected]"
                },
                {
                    "name": "sigco",
                    "value": "Company"
                },
                {
                    "name": "mobile",
                    "value": "0760000000"
                }
            ]
        },
        {
            "fields": [
                {
                    "name": "fstname",
                    "value": ""
                },
                {
                    "name": "sndname",
                    "value": ""
                },
                {
                    "name": "email",
                    "value": ""
                },
                {
                    "name": "mobile",
                    "value": ""
                },
                {
                    "name": "sigco",
                    "value": ""
                }
            ]
        }
    ]
}
*/


/* And here's the result I'm trying to achieve, with the value for "fstname" in the second fields array is set to "New name"

{
    "id": "6988",
    "signatories": [
        {
            "fields": [
                {
                    "name": "fstname",
                    "value": "Bruce"
                },
                {
                    "name": "sndname",
                    "value": "Lee"
                },
                {
                    "name": "email",
                    "value": "[email protected]"
                },
                {
                    "name": "sigco",
                    "value": "Company"
                },
                {
                    "name": "mobile",
                    "value": "0760000000"
                }
            ]
        },
        {
            "fields": [
                {
                    "name": "fstname",
                    "value": "New name"
                },
                {
                    "name": "sndname",
                    "value": ""
                },
                {
                    "name": "email",
                    "value": ""
                },
                {
                    "name": "mobile",
                    "value": ""
                },
                {
                    "name": "sigco",
                    "value": ""
                }
            ]
        }
    ]
}
*/