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.

I think the easiest way to go would be to decode the json string first, do all the transformations on the scala objects directly and encode the result back to json again. — Florian Baierl
– Florian Baierl, Commented Oct 8, 2018 at 9:48
Apart from asking a perfectly valid question, try to read into documentation and attempt to solve your problem before coming to StackOverflow. Once you come to the point of asking a question here, list what you have looked into and any code snippets that you wrote while doing so, even if they don't work. StackOverflow doesn't write your code, it tries to help you where you are stuck. This is not to annoy you, but to get you to learn to do it yourself. — Markus Appel
– Markus Appel, Commented Oct 8, 2018 at 14:43

Markus Appel · Accepted Answer · 2018-10-09 10:20:38Z

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).

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