Decode json inside json string

Question

I'm dealing with an API which expects a JSON object where one of the values (blob) is a JSON object stringified:

{
    "credential": {
        "blob": "{\"access\":\"181920\",\"secret\":\"secretKey\"}",
        "project_id": "731fc6f265cd486d900f16e84c5cb594",
        "type": "ec2",
        "user_id": "bb5476fd12884539b41d5a88f838d773"
    }
}

My domain class is:

case class Credential(access: String, secret: String, projectId: String, userId: String)

Encoding the domain class is easy:

implicit val encoder: Encoder[Credential] = (a: Credential) => Json.obj(
  "type" -> "ec2".asJson,
  "blob" -> Map("access" -> a.access, "secret" -> a.secret).asJson.noSpaces.asJson,
  "project_id" -> a.projectId.asJson,
  "user_id" -> a.userId.asJson
)

However decoding is much harder:

implicit val decoder: Decoder[Credential] = (c: HCursor) => for {
  blobJsonString <- c.get[String]("blob")
  blob <- decode[Json](blobJsonString).left.map(e => DecodingFailure(e.getMessage, c.downField("blob").history))
  access <- blob.hcursor.get[String]("access")
  secret <- blob.hcursor.get[String]("secret")
  projectId <- c.get[String]("project_id")
  userId <- c.get[String]("user_id")
} yield Credential(access, secret, projectId, userId)

I don't like this implementation because it forces me to depend on circe-parser, and to break the abstraction layer the Encoders/Decoders provide.

Is there a way to implement a Decoder which does double decoding in a general way?

Answer 1

Well, because described JSON is not really typical case, I'm not sure if it is possible to completely avoid manual parsing, but if you will change case class representing this structure, you can leverage some advantages, which circe offers. Please, find code example below:

import io.circe._
import io.circe.generic.semiauto._
import io.circe.generic.auto._

object CredentialsParseApp {
  case class CredentialsBlob(access: String, secret: String)

  object CredentialsBlob {

    implicit val encoder: Encoder[CredentialsBlob] = {
      val derivedEncoder: Encoder[CredentialsBlob] = deriveEncoder[CredentialsBlob]
      Encoder[String].contramap(blob => derivedEncoder(blob).noSpaces)
    }

    implicit val decoder: Decoder[CredentialsBlob] = {
      val derivedDecoder: Decoder[CredentialsBlob] = deriveDecoder[CredentialsBlob]
      Decoder[String].emap { value =>
        for {
          json <- parser.parse(value).left.map(_.message)
          blob <- json.as(derivedDecoder).left.map(_.message)
        } yield blob
      }
    }
  }

  case class Credentials(blob: CredentialsBlob, project_id: String, `type`: String = "ec2", user_id: String)
  case class Response(credential: Credentials)

  def main(args: Array[String]): Unit = {
    val jsonString =
      """{
         |    "credential": {
         |        "blob": "{\"access\": \"181920\", \"secret\": \"secretKey\" }",
         |        "project_id": "731fc6f265cd486d900f16e84c5cb594",
         |        "type": "ec2",
         |        "user_id": "bb5476fd12884539b41d5a88f838d773"
         |    }
         |}""".stripMargin

    println(parser.parse(jsonString).flatMap(_.as[Response]))
  }
}

which in my case produced next result:

Right(Response(Credentials(CredentialsBlob(181920,secretKey),731fc6f265cd486d900f16e84c5cb594,ec2,bb5476fd12884539b41d5a88f838d773)))

I used circe version "0.12.3" for this example. Hope this helps!