Given the JSON...
[ {"ID": "foo"}, {"ID": "bar"} ]
Represented with case classes...
case class Example(models: List[Model])
case class Model(id: String)
I attempt the following which fails with overloaded method value read with alternatives.
trait JsonReader {
implicit val modelReads: Reads[Model] = (__ \ "name").read[String](Model)
implicit val exampleReads: Reads[Example] = JsPath.read[List[Model]](Example)
def get (response: Response) = response.json.as[Example]
}
What is the correct way to parse this?
For a strange reason I did not find an elegant solution to read a json model with only one value. For 2 and more values you may write:
implicit val reader = (
(__ \ 'id).read[Long] and
(__ \ 'field1).read[String] and
(__ \ 'field2).read[String])(YourModel.apply _)
For a json with 1 field try using something like that:
implicit val reader = new Reads[Model] {
def reads(js: JsValue): JsResult[Model] = {
JsSuccess(Model((js \ "name").as[String]))
}
}
This should work but doesn't look nice :(
Related
Let's say I've got a reads that creates an object from JSON with two optional fields:
implicit val rd: Reads[MyObject] = (
(__ \ "field1").readNullable[String] and
(__ \ "field2").readNullable[String]
)(MyObject.apply _)
I want to check to make sure that the value of field1 is one of the values in the list:
List("foo", "bar")
I can do that after the fact, by creating a new MyObject and mapping the values through a function to transform them, but I feel like there should be a way to do this more elegantly using JSON transformers or something.
Ideally, I want the Reads to read the nullable value of field1 and transform it if it is defined, without the need to post-process it. Is there some way of sneaking a transform in there?
You can use this approach:
case class MyObject(a: Option[String], b: Option[String])
val allowedValues = Seq("foo", "bar")
implicit val reads: Reads[MyObject] = new Reads[MyObject] {
override def reads(json: JsValue): JsResult[MyObject] = {
val a = (json \ "a").asOpt[String].filter(allowedValues.contains)
val b = (json \ "b").asOpt[String]
JsSuccess(MyObject(a, b))
}
}
Usage examples:
scala> Json.parse(""" { "a": "bar", "b": "whatever"} """).validate[MyObject]
res2: play.api.libs.json.JsResult[MyObject] = JsSuccess(MyObject(Some(bar),Some(whatever)),)
scala> Json.parse(""" { "a": "other", "b": "whatever"} """).validate[MyObject]
res3: play.api.libs.json.JsResult[MyObject] = JsSuccess(MyObject(None,Some(whatever)),)
scala> Json.parse(""" {} """).validate[MyObject]
res4: play.api.libs.json.JsResult[MyObject] = JsSuccess(MyObject(None,None),)
Okay, after doing some more research, I came up with the following:
In play.api.libs.json.ConstraintReads there is a function called verifying(cond: A => Boolean) that returns a Reads[A]. This can be passed as a parameter to JsPath.readNullable[A] like so:
implicit val rd: Reads[MyObject] = (
(__ \ "field1").readNullable[String](verifying(allowedValues.contains)) and
(__ \ "field2").readNullable[String]
)(MyObject.apply _)
This will return a JsResponse, either a JsSuccess if "field1" validates, or a JsError if it doesn't validate. It actually fails on an invalid input, rather than just ignoring the input. That's more like the behaviour I wanted.
There are a number of other constraint functions that perform similar tests on the read value, as well.
Play's JSON serialization is by default permissive when serializing from JSON into a case class. For example.
case class Stuff(name: String, value: Option[Boolean])
implicit val stuffReads: Reads[Stuff] = (
( __ \ 'name).read[String] and
( __ \ 'value).readNullable[Boolean]
)(Stuff.apply _)
If the following JSON was received:
{name: "My Stuff", value: true, extraField: "this shouldn't be here"}
It will succeed with a 'JsSuccess' and discard the 'extraField'.
Is there a way to construct the Json Reads function to have it return a JsError if there are 'unhandled' fields?
You can verify that the object doesn't contain extra keys before performing your own decoding:
import play.api.data.validation.ValidationError
def onlyFields(allowed: String*): Reads[JsObject] = Reads.filter(
ValidationError("One or more extra fields!")
)(_.keys.forall(allowed.contains))
Or if you don't care about error messages (and that one's not very helpful, anyway):
def onlyFields(allowed: String*): Reads[JsObject] =
Reads.verifying(_.keys.forall(allowed.contains))
And then:
implicit val stuffReads: Reads[Stuff] = onlyFields("name", "value") andThen (
(__ \ 'name).read[String] and
(__ \ 'value).readNullable[Boolean]
)(Stuff)
The repetition isn't very nice, but it works.
Inspired from Travis' comment to use LabelledGeneric I was able achieve compile time safe solution.
object toStringName extends Poly1 {
implicit def keyToStrName[A] = at[Symbol with A](_.name)
}
case class Foo(bar: String, boo: Boolean)
val labl = LabelledGeneric[Foo]
val keys = Keys[labl.Repr].apply
now keys.map (toStringName).toList will give you
res0: List[String] = List(bar, boo)
Is there a way to perform conditional logic while parsing json using Scala/Play?
For example, I would like to do something like the following:
implicit val playlistItemInfo: Reads[PlaylistItemInfo] = (
(if(( (JsPath \ "type1").readNullable[String]) != null){ (JsPath \ "type1" \ "id").read[String]} else {(JsPath \ "type2" \ "id").read[String]}) and
(JsPath \ "name").readNullable[String]
)(PlaylistItemInfo.apply _)
In my hypothetical JSON parsing example, there are two possible ways to parse the JSON. If the item is of "type1", then there will be a value for "type1" in the JSON. If this is not present in the JSON or its value is null/empty, then I would like to read the JSON node "type2" instead.
The above example does not work, but it gives you the idea of what I am trying to do.
Is this possible?
The proper way to do this with JSON combinators is to use orElse. Each piece of the combinator must be a Reads[YourType], so if/else doesn't quite work because your if clause doesn't return a Boolean, it returns Reads[PlaylistItemInfo] checked against null which will always be true. orElse let's us combine one Reads that looks for the type1 field, and a second one that looks for the type2 field as a fallback.
This might not follow your exact structure, but here's the idea:
import play.api.libs.json._
import play.api.libs.functional.syntax._
case class PlaylistItemInfo(id: Option[String], tpe: String)
object PlaylistItemInfo {
implicit val reads: Reads[PlaylistItemInfo] = (
(__ \ "id").readNullable[String] and
(__ \ "type1").read[String].orElse((__ \ "type2").read[String])
)(PlaylistItemInfo.apply _)
}
// Read type 1 over type 2
val js = Json.parse("""{"id": "test", "type1": "111", "type2": "2222"}""")
scala> js.validate[PlaylistItemInfo]
res1: play.api.libs.json.JsResult[PlaylistItemInfo] = JsSuccess(PlaylistItemInfo(Some(test),111),)
// Read type 2 when type 1 is unavailable
val js = Json.parse("""{"id": "test", "type2": "22222"}""")
scala> js.validate[PlaylistItemInfo]
res2: play.api.libs.json.JsResult[PlaylistItemInfo] = JsSuccess(PlaylistItemInfo(Some(test),22222),)
// Error from neither
val js = Json.parse("""{"id": "test", "type100": "fake"}""")
scala> js.validate[PlaylistItemInfo]
res3: play.api.libs.json.JsResult[PlaylistItemInfo] = JsError(List((/type2,List(ValidationError(error.path.missing,WrappedArray())))))
Play's JSON serialization is by default permissive when serializing from JSON into a case class. For example.
case class Stuff(name: String, value: Option[Boolean])
implicit val stuffReads: Reads[Stuff] = (
( __ \ 'name).read[String] and
( __ \ 'value).readNullable[Boolean]
)(Stuff.apply _)
If the following JSON was received:
{name: "My Stuff", value: true, extraField: "this shouldn't be here"}
It will succeed with a 'JsSuccess' and discard the 'extraField'.
Is there a way to construct the Json Reads function to have it return a JsError if there are 'unhandled' fields?
You can verify that the object doesn't contain extra keys before performing your own decoding:
import play.api.data.validation.ValidationError
def onlyFields(allowed: String*): Reads[JsObject] = Reads.filter(
ValidationError("One or more extra fields!")
)(_.keys.forall(allowed.contains))
Or if you don't care about error messages (and that one's not very helpful, anyway):
def onlyFields(allowed: String*): Reads[JsObject] =
Reads.verifying(_.keys.forall(allowed.contains))
And then:
implicit val stuffReads: Reads[Stuff] = onlyFields("name", "value") andThen (
(__ \ 'name).read[String] and
(__ \ 'value).readNullable[Boolean]
)(Stuff)
The repetition isn't very nice, but it works.
Inspired from Travis' comment to use LabelledGeneric I was able achieve compile time safe solution.
object toStringName extends Poly1 {
implicit def keyToStrName[A] = at[Symbol with A](_.name)
}
case class Foo(bar: String, boo: Boolean)
val labl = LabelledGeneric[Foo]
val keys = Keys[labl.Repr].apply
now keys.map (toStringName).toList will give you
res0: List[String] = List(bar, boo)
I'm trying to write a custom Json serializer in play for a case class but I don't want it to serialize all the fields of the class. I'm pretty new to Scala, so that is surely the problem but this is what I tried so far:
case class Foo(a: String, b: Int, c: Double)
Now the default way of doing this, as far as I saw in the examples is:
implicit val fooWrites: Writes[Foo] = (
(__ \ "a").write[String] and
(__ \ "b").write[Int]
(__ \ "c").write[Double]
) (unlift(Foo.unapply))
But what if I want to omit "c" from the Json output? I've tried this so far but it doesn't compile:
implicit val fooWritesAlt: Writes[Foo] = (
(__ \ "a").write[String] and
(__ \ "b").write[Int]
) (unlift({(f: Foo) => Some((f.a, f.b))}))
Any help is greatly appreciated!
If you are using Playframework 2.2 (not sure about earlier versions, but it should work as well) try this:
implicit val writer = new Writes[Foo] {
def writes(foo: Foo): JsValue = {
Json.obj("a" -> foo.a,
"b" -> foo.b)
}
}
What I usually do is convert a field to None and use writeNullable on it:
implicit val fooWrites: Writes[Foo] = (
(__ \ "a").write[String] and
(__ \ "b").write[Int]
(__ \ "c").writeNullable[Double].contramap((_: Double) => None)
) (unlift(Foo.unapply))
Instead of specifying a combinator which produces an OWrites instance, one can directly construct an OWrites just as well:
val ignore = OWrites[Any](_ => Json.obj())
implicit val fooWrites: Writes[Foo] = (
(__ \ "a").write[String] and
(__ \ "b").write[Int] and
ignore
) (unlift(Foo.unapply))
This will ignore any value of the case class at that position and simply always return an empty JSON object.
This is very easy to do with the Play's JSON transformers:
val outputFoo = (__ \ 'c).json.prune
and then apply transform(outputFoo) to your existing JsValue:
val foo: Foo
val unprunedJson: JsValue = Json.toJson(foo)
unprunedJson.transform(outputFoo).map { jsonp =>
Ok(Json.obj("foo" -> jsonp))
}.recoverTotal { e =>
BadRequest(JsError.toFlatJson(e))
}
see here http://mandubian.com/2013/01/13/JSON-Coast-to-Coast/
What version of Play are you using? fooWritesAlt compiles for me using 2.1.3. One thing to note is that I needed to explicitly use the writes object to write the partial JSON object, i.e.
fooWritesAt.writes(Foo("a", 1, 2.0))
returns
{"a": "a", "b": 2}