Modifying a JSON array in Scala with circe

Question

I have a JSON string with array as the following:

{
"cars": {
    "Nissan": [
        {"model":"Sentra", "doors":4},
        {"model":"Maxima", "doors":4}
    ],
    "Ford": [
        {"model":"Taurus", "doors":4},
        {"model":"Escort", "doors":2}
    ]
}
}

I would like to edit a new card brand, using circe at scala.
Instead of

"Nissan": [
        {"model":"Sentra", "doors":4},
        {"model":"Maxima", "doors":4},
    ]

I would like to have as a result:

"Nissan": [
        {"model":"Sentra", "doors":1000},
    ],

Thanks.

Answer 1

Made after reading the manual, please do the same next time!

Apart from your JSON being invalid (Trailing comma on object in array "Nissan"), it should look something like this:

import cats.syntax.either._
import io.circe._, io.circe.parser._

val json: String = """
{
"cars": {
    "Nissan": [
        {"model":"Sentra", "doors":4},
        {"model":"Maxima", "doors":4}
    ],
    "Ford": [
        {"model":"Taurus", "doors":4},
        {"model":"Escort", "doors":2}
    ]
}
}
"""

val newJson = parse(json).toOption
  .flatMap { doc =>
    doc.hcursor
      .downField("cars")
      .downField("Nissan")
      .withFocus(_ =>
          Json.arr(
            Json.fromFields(
              Seq(
                ("model", Json.fromString("Sentra")),
                ("doors", Json.fromInt(1000))
              )
            )
          )
      )
      .top
  }

newJson match {
  case Some(v) => println(v.toString)
  case None => println("Failure!")
}

Try it out! (Rerun to see correct indentation!)

---

newJson is actually an Option, so if parsing or modifying fails, you will get None.

Calling toOption on parse(json) converts the returned Either[Json] (parsing succeeded / failed) to an Option[Json].

We need to use an Option[Json] in the first place because .top returns a Option[Json] (modifying succeeded / failed), too.

This way we can flatMap and don't have to deal with any nested types (see here).