Mike Limansky - Easy JSON analyze with spray-json

If you use Spray or Akka HTTP to create a REST service, possibly you need to work with JSON objects. Both Spray and Akka HTTP have built-in support of spray-json.

In most cases you have a fixed set of fields in your JSON API, so the proposed way to work with spray-json is to create model case classes and marshallers/unmarshallers for them:

case class Person(name: String, age: Int, title: Option[String])

trait PersonRoutes extends DefaultJsonProtocol with SprayJsonSupport {
  implicit val personFormat = jsonFormat3(Person)

  val route =
    put {
      path("people") {
        entity(as[Person]) { person =>
          // Store to db
          compete(StatusCodes.Created)
        }
      }
    }
}

In this example the implicit formatter does all JSON validation and deserialization work. So person is an instance of the class Person. But sometimes you might not have a strict model, but just some JSON object. You can represent this data as an instance of JsObject class which is defined as:

case class JsObject(fields: Map[String, JsValue]) extends JsValue

So, if we’d like to add some schemeless information to our Person class we can just add a JsObject field.

case class Person(name: String, age: Int, title: Option[String], extras: JsObject)

The problem is that this is not really useful to inspect JsObject. The only thing you can do with it is to get its fields like a Map[String, JsValue].

For example some of the Person objects have address information:

{
    "name" : "John Doe",
    "age" : 42,
    "extras" : {
        "address" : {
            "city"  : "Moscow",
            "street" : "Zemlyanoy Val"
        }
    }
}

Let’s say we’d like to inspect these objects and pass only if the city is Moscow. To do that we can write something like:

entity(as[Person]) { person =>
  val city = for {
    addr <- person.extras.fields.get("address")
    ao   <- Try(addr.asJsObject).toOption
    c    <- ao.fields.get("city")
  } yield c

  if (city.map(_ == "Moscow").getOrElse(false)) {
    complete(StatusCodes.Ok)
  } else {
    complete(StatusCodes.BadRequest)
  }
}

This code looks quite ugly. Even with two layers we have a lot of boilerplate steps: extract a JsValue, convert it to a JsObject, extract the next value, etc. It would be nice to have DSL for traversing through the objects. I think it can be similar to XPath:

entity(as[Person]) { person =>
  if (person / "address" / "city" === "Moscow") {
    complete(StatusCodes.Ok)
  } else {
    complete(StatusCodes.BadRequest)
  }
}

Let’s create an implicit class to extend JsObject:

implicit class JsObjectOps(val o: JsObject) extends AnyVal {
  def / (name: String) = ???
}

But what should it return? We can return a JsValue, but there are several problems:

The object might not contain the field we are looking for. So, we need at least an Option[JsValue].
We’d like to chain path elements to create more complex paths.
We need to have === and =!= operators to check the returned values.

To meet all of these requirements, we need to create another class, which will wrap the Option[JsValue]:

class JsFieldOps(val field: Option[JsValue]) {
  def /(name: string) = field map (_ / name) getOrElse this
  def ===(x: JsValue) = field.contains(x)
  def =!=(x: JsValue) = !field.contains(x)
}

The implementation of === and =!= is quite obvious. We just check the values in the underlying field. The most interesting part is the / method (but don’t get too excited – this one is not rocket science either :)). There are two cases. If the field is empty we can just return the same empty object. But if not, we can apply the same / we used initially to create this object (that’s the part I left not implemented yet in the very beginning of the implementation). So it looks like we need to extend the JsValue, not the JsObject to add /, but it has to be applicable only to the objects. There is a method called asJsObject in the JsObject class, which throws an exception if the class is not a JsObject. Thus, the implementation of the JsValueOps (instead of the JsObjectOps) will be like:

import scala.util.Try
import spray.json.JsValue

implicit class JsValueOps(val value: JsValue) extends AnyVal {
  def /(name: String) =
  JsFieldOps(Try(value.asJsObject).toOption.flatMap(_.fields.get(name)))
}

And this is a complete implementation of simple DSL for querying values in JsObjects.

P.S. there is a great library called json-lenses which provides a more powerful way to query and update JSON objects. It gives you objects called “lens”, which encapsulate a path through a JSON object, and allows you to get and set values (of course set means create a modified object, because JsObject is immutable).

Easy JSON analyze with spray-json

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