Parsing an object-sequence with Play-JSON library

Question

As a disclaimer: I'm very new to Scala and functional programming in general.

I have the following classes:

case class A(
  field1: String,
  field2: DateTime
)

case class B(
  listOfStuff: Seq[A]
)

object A{
  def create(field1: String, field2: DateTime): A = A(field1, field2)
}

object B{
  def create(listOfStuff: Seq[A]): B = B(listOfStuff)
}

(The create() functions exist because I sometimes had issues in my code when using apply(). Let's ignore this, I doubt it's relevant.)

I get these objects in JSON format and I try to parse them using the Play-JSON library. An important aspect is that the list (Seq) can be missing from the JSON text, it's an optional field.

With that in mind, this is how I wrote that particular line:

private implicit val messageReader = (
    //...
    (__ \ "stuff").readNullable[Seq[A]].map(_.getOrElse(Seq()))
    //...
)(B.create _)

When compiling, I get the following error:

Error:(!line!, !column!) No Json deserializer found for type Seq[A]. Try to implement an implicit Reads or Format for this type.

From what I saw in this question, apparently you need to have an implicit instance of Reads[T] for every type T that is not part of the language (or something like that, did I mention I'm new to this?)

So I also added a secondary Reads[T] in the same scope, my code now looked like this:

private implicit val messageReader = (
    (__ \ "stuff").readNullable[Seq[A]].map(_.getOrElse(Seq()))
)(B.create _)

private implicit val anotherReader = (
    (__ \ "field1").read[String] and
    (__ \ "field2").read[String].map(org.joda.time.DateTime.parse)
)(A.create _)

However I still get that same error on the same line.

I feel like there's some very easy and obvious problem here, but I can't figure out what it is.

How do I fix this?

To be able to use parsers of Seq[T], parsers for T must be available — cchantep
– cchantep, Commented Jul 21, 2017 at 15:07
@cchantep Which is why I added the private implicit val anotherReader. It still doesn't work. — Radu Murzea
– Radu Murzea, Commented Jul 21, 2017 at 15:19
@cchantep Your suggestion worked. I simply reversed their order and everything was fine. Post it as an answer and I will accept it ;) . — Radu Murzea
– Radu Murzea, Commented Jul 24, 2017 at 7:31

tgk · Accepted Answer · 2017-07-23 06:23:24Z

0

if B.create accepts a single argument then its reads can look like this:

private implicit val messageReader : Reads[B] =
  (__ \ "stuff").readNullable[Seq[A]].map(_.getOrElse(Seq())).map(B.create)

edited Jul 23, 2017 at 6:23

tgk

4,2262 gold badges31 silver badges49 bronze badges

answered Jul 22, 2017 at 7:42

LiorH

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1 Comment

Radu Murzea Over a year ago

Actually they're about 20 fields there, but of course I deleted them for the purpose of this question.

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