I have a case class
case class Foo(bar: Int, baz: String)
and I am trying to write a JSON serialization for it. But I have an additional requirement, to add one more field to the generated Json (say "greeting": "Hello")
I am trying something along these lines:
val writes = ((JsPath \ "bar").write[Int] and
(JsPath \ "baz").write[String] and
(JsPath \ "greeting").write[String])(unlift(Foo.unapply))
But, how should I pass the static "Hello" string to the above Writes?
And how can I use this writes to create a Format for my Foo class?
I would do it like this:
val writes = Writes[Foo](f => {
Json.obj(
"bar" -> f.bar,
"baz" -> f.baz,
"greeting" -> "Hello")
})
Drawback is that you have to specify the members twice, but it's handy for special cases like this where you need additional control.
In a one-off scenario, where you don't want to define yet another Writes, you could do this:
Json.toJson(foo).as[JsObject] ++ Json.obj("greeting" -> "hello")
I have ended up with another solution:
val writes = ((JsPath \ "bar").write[Int] and
(JsPath \ "baz").write[String] and
(JsPath \ "greeting").write[String])((f: Foo) => (f.bar, f.baz, "Hello"))
Related
I am unable to understand example code of JsPath and Read in the documentation
https://www.playframework.com/documentation/2.2.1/ScalaJsonCombinators
import play.api.libs.json._
import play.api.libs.functional.syntax._
Question 1 - We create a custom reader. Reads should be able to read a structure of data consisting of String, Float and a List. But in the example below, we pass it a Json! How is the Json getting converted to (String, Fload and List)?
Question 2 - we use JsPath \ "key1" but where have we passed the JSON?
val customReads: Reads[(String, Float, List[String])] =
(JsPath \ "key1").read[String](email keepAnd minLength(5)) and
(JsPath \ "key2").read[Float](min(45)) and
(JsPath \ "key3").read[List[String]]
tupled
import play.api.libs.json.Json
val js = Json.obj(
"key1" -> "alpha",
"key2" -> 123.345F,
"key3" -> Json.arr("alpha", "beta")
)
res5: JsSuccess(("alpha", 123.345F, List("alpha", "beta")))
scala> customReads.reads(js)
customReads.reads(js).fold(
invalid = { errors => ... },
valid = { res =>
val (s, f, l): (String, Float, List[String]) = res
...
}
)
Question 1
We create a custom reader.
Yes
Reads should be able to read a structure of data consisting of String, Float and a List.
No
val customReads: Reads[(String, Float, List[String])]= ...
Means that customReads is the variable of type Reads, and that Reads.reads must return tuple of types (String, Float, List[String])
But in the example below, we pass it a JSON! How is the JSON getting converted to (String, Float and List)?
Reads.reads is a function that takes JSON as a parameter and returns some value, extracted from the JSON by defined rules. In our case, the rules are:
(JsPath \ "key1").read[String](email keepAnd minLength(5))
and
(JsPath \ "key2").read[Float](min(45))
and
(JsPath \ "key3").read[List[String]]
all these values must be
tupled
so our Reads.reads function returns (String, Float, List[String]) tuple.
Question 2
we use JsPath \ "key1" but where have we passed the JSON?
JsPath \ "key1" is not the actual code to process the JSON, it's a rule to process a JSON. i.e it is like XPath expression /key1
Lets imagine I have a case class like this:
case class Product(ean: Long, name: String, description: String)
and I want so serialize objects of this class to Json, I can implement the Writes trait like this:
implicit val productWrites: Writes[Product] = (
(JsPath \ "ean").write[Long] and
(JsPath \ "name").write[String] and
(JsPath \ "description").write[String]
)(unlift(Product.unapply))
This works fine if I want to serialize all the attributes of the object. Now lets say I don't want to serialize the ean. I tried something like this:
implicit val productWrites: Writes[Product] = (
(JsPath \ "name").write[String] and
(JsPath \ "description").write[String]
)(unlift(Product.unapply))
This doesn't seem to work since one needs to use all the fields/attributes that the unapply method returns.
Is there a way to make the second serialization method work with only the attributes that I want to serialize or do I have to use something like this:
implicit object ProductWrites extends Writes[Product] {
def writes(p: Product) = Json.obj(
"name" -> Json.toJson(p.name),
"description" -> Json.toJson(p.description)
)
}
Is this the only way?
unlift(Product.unapply) has a type Product => (Long, String, String).
In this case, the argument should have a type Product => (String, String). You can write a function literal like following.
implicit val productWrites: Writes[Product] = (
(JsPath \ "name").write[String] and
(JsPath \ "description").write[String]
)(p => (p.name, p.description))
I think your last example is the way to go. Here's another way of doing the same thing using an implicit val instead of an implicit object:
implicit val productWrites: Writes[Product] = Writes { p =>
Json.obj(
"name" -> Json.toJson(p.name),
"description" -> Json.toJson(p.description)
)
}
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())))))
So, I've just recently started learning Scala. Sorry for my incompetence in advance.
I tried to look up my answer on stackoverflow. I was able to find several related topics, but I didn't spot my problem.
I'm trying to send a json response based on a Scala object. I have an Action and I'm doing the following:
def oneCredential = Action {
val cred = Credential("John", "Temp", "5437437")
Ok(Json.toJson(cred))
}
I've created a case class and appropriate implicit Writes[T] for it
import play.api.libs.json._
import play.api.libs.functional.syntax._
import play.api.libs.json.util._
case class Credential(name: String, account: String, password: String)
object Credential{
implicit val credentialWrites = (
(__ \ "name").write[String] and
(__ \ "account").write[String] and
(__ \ "password").write[String]
)(Credential)
}
When I'm trying to run this, I've the following error: "Overloaded method value [apply] cannot be applied to (models.Credential.type)". Also, I tried this
implicit val credentialWrites = (
(__ \ "name").write[String] and
(__ \ "account").write[String] and
(__ \ "password").write[String]
)(Credential.apply _)
Fail. The error: could not find implicit value for parameter fu: play.api.libs.functional.Functor[play.api.libs.json.OWrites]
Then this:
implicit val credentialWrites = (
(__ \ "name").writes[String] and
(__ \ "account").writes[String] and
(__ \ "password").writes[String]
)(Credential)
Another fail: "value writes is not a member of play.api.libs.json.JsPath Note: implicit value credentialWrites is not applicable here because it comes after the application point and it lacks an explicit result type". Right, I understood the first part of an error, but not the second.
Finally I found a shorthand solution:
implicit val credentialWrites = Json.writes[Credential]
With this I've got no errors and the code finally worked. I've found the solution on this blog. It's said that the shorthand form is exactly the same as the one with "writes" above. But this "long" form didn't work for me.
Why is shorthand version working, while the long one isn't? Can somebody explain this?
Thank you!
PS Scala version: 2.10.2
The definitions you've given would work for Reads, but Writes needs a different kind of argument at the end. Take the following example:
case class Baz(foo: Int, bar: String)
val r = (__ \ 'foo).read[Int] and (__ \ 'bar).read[String]
val w = (__ \ 'foo).write[Int] and (__ \ 'bar).write[String]
r can be applied to a function (Int, String) => A to get a Reads[A], which means we can use it as follows (these are all equivalent):
val bazReader1 = r((foo: Int, bar: String) => Baz(foo, bar))
val bazReader2 = r(Baz.apply _)
val bazReader3 = r(Baz)
What we're doing is lifting the function into the applicative functor for Reads so that we can apply it to our Reads[Int] and Reads[String] (but you don't need to care about that if you don't want to).
w takes a different kind of argument (again, you don't need to care, but this is because Writes has a contravariant functor—it doesn't have an applicative functor):
val bazWriter1 = w((b: Baz) => (b.foo, b.bar))
We could write this equivalently as the following:
val bazWriter2 = w(unlift(Baz.unapply))
Here we're using the case class's automatically generated extractor, unapply, which returns an Option[(Int, String)]. We know in this case that it'll always return a Some, so we can use unlift (which comes from the functional syntax package, and just calls the standard library's Function.unlift) to turn the Baz => Option[(Int, String)] into the required Baz => (Int, String).
So just change your final line to )(unlift(Credential.unapply)) and you're good to go.
I have an equivalent of the following model in play scala :
case class Foo(id:Int,value:String)
object Foo{
import play.api.libs.json.Json
implicit val fooFormats = Json.format[Foo]
}
For the following Foo instance
Foo(1, "foo")
I would get the following JSON document:
{"id":1, "value": "foo"}
This JSON is persisted and read from a datastore. Now my requirements have changed and I need to add a property to Foo. The property has a default value :
case class Foo(id:String,value:String, status:String="pending")
Writing to JSON is not a problem :
{"id":1, "value": "foo", "status":"pending"}
Reading from it however yields a JsError for missing the "/status" path.
How can I provide a default with the least possible noise ?
(ps: I have an answer which I will post below but I am not really satisfied with it and would upvote and accept any better option)
Play 2.6+
As per #CanardMoussant's answer, starting with Play 2.6 the play-json macro has been improved and proposes multiple new features including using the default values as placeholders when deserializing :
implicit def jsonFormat = Json.using[Json.WithDefaultValues].format[Foo]
For play below 2.6 the best option remains using one of the options below :
play-json-extra
I found out about a much better solution to most of the shortcomings I had with play-json including the one in the question:
play-json-extra which uses [play-json-extensions] internally to solve the particular issue in this question.
It includes a macro which will automatically include the missing defaults in the serializer/deserializer, making refactors much less error prone !
import play.json.extra.Jsonx
implicit def jsonFormat = Jsonx.formatCaseClass[Foo]
there is more to the library you may want to check: play-json-extra
Json transformers
My current solution is to create a JSON Transformer and combine it with the Reads generated by the macro. The transformer is generated by the following method:
object JsonExtensions{
def withDefault[A](key:String, default:A)(implicit writes:Writes[A]) = __.json.update((__ \ key).json.copyFrom((__ \ key).json.pick orElse Reads.pure(Json.toJson(default))))
}
The format definition then becomes :
implicit val fooformats: Format[Foo] = new Format[Foo]{
import JsonExtensions._
val base = Json.format[Foo]
def reads(json: JsValue): JsResult[Foo] = base.compose(withDefault("status","bidon")).reads(json)
def writes(o: Foo): JsValue = base.writes(o)
}
and
Json.parse("""{"id":"1", "value":"foo"}""").validate[Foo]
will indeed generate an instance of Foo with the default value applied.
This has 2 major flaws in my opinion:
The defaulter key name is in a string and won't get picked up by a refactoring
The value of the default is duplicated and if changed at one place will need to be changed manually at the other
The cleanest approach that I've found is to use "or pure", e.g.,
...
((JsPath \ "notes").read[String] or Reads.pure("")) and
((JsPath \ "title").read[String] or Reads.pure("")) and
...
This can be used in the normal implicit way when the default is a constant. When it's dynamic, then you need to write a method to create the Reads, and then introduce it in-scope, a la
implicit val packageReader = makeJsonReads(jobId, url)
An alternative solution is to use formatNullable[T] combined with inmap from InvariantFunctor.
import play.api.libs.functional.syntax._
import play.api.libs.json._
implicit val fooFormats =
((__ \ "id").format[Int] ~
(__ \ "value").format[String] ~
(__ \ "status").formatNullable[String].inmap[String](_.getOrElse("pending"), Some(_))
)(Foo.apply, unlift(Foo.unapply))
I think the official answer should now be to use the WithDefaultValues coming along Play Json 2.6:
implicit def jsonFormat = Json.using[Json.WithDefaultValues].format[Foo]
Edit:
It is important to note that the behavior differs from the play-json-extra library. For instance if you have a DateTime parameter that has a default value to DateTime.Now, then you will now get the startup time of the process - probably not what you want - whereas with play-json-extra you had the time of the creation from the JSON.
I was just faced with the case where I wanted all JSON fields to be optional (i.e. optional on user side) but internally I want all fields to be non-optional with precisely defined default values in case the user does not specify a certain field. This should be similar to your use case.
I'm currently considering an approach which simply wraps the construction of Foo with fully optional arguments:
case class Foo(id: Int, value: String, status: String)
object FooBuilder {
def apply(id: Option[Int], value: Option[String], status: Option[String]) = Foo(
id getOrElse 0,
value getOrElse "nothing",
status getOrElse "pending"
)
val fooReader: Reads[Foo] = (
(__ \ "id").readNullable[Int] and
(__ \ "value").readNullable[String] and
(__ \ "status").readNullable[String]
)(FooBuilder.apply _)
}
implicit val fooReader = FooBuilder.fooReader
val foo = Json.parse("""{"id": 1, "value": "foo"}""")
.validate[Foo]
.get // returns Foo(1, "foo", "pending")
Unfortunately, it requires writing explicit Reads[Foo] and Writes[Foo], which is probably what you wanted to avoid? One further drawback is that the default value will only be used if the key is missing or the value is null. However if the key contains a value of the wrong type, then again the whole validation returns a ValidationError.
Nesting such optional JSON structures is not a problem, for instance:
case class Bar(id1: Int, id2: Int)
object BarBuilder {
def apply(id1: Option[Int], id2: Option[Int]) = Bar(
id1 getOrElse 0,
id2 getOrElse 0
)
val reader: Reads[Bar] = (
(__ \ "id1").readNullable[Int] and
(__ \ "id2").readNullable[Int]
)(BarBuilder.apply _)
val writer: Writes[Bar] = (
(__ \ "id1").write[Int] and
(__ \ "id2").write[Int]
)(unlift(Bar.unapply))
}
case class Foo(id: Int, value: String, status: String, bar: Bar)
object FooBuilder {
implicit val barReader = BarBuilder.reader
implicit val barWriter = BarBuilder.writer
def apply(id: Option[Int], value: Option[String], status: Option[String], bar: Option[Bar]) = Foo(
id getOrElse 0,
value getOrElse "nothing",
status getOrElse "pending",
bar getOrElse BarBuilder.apply(None, None)
)
val reader: Reads[Foo] = (
(__ \ "id").readNullable[Int] and
(__ \ "value").readNullable[String] and
(__ \ "status").readNullable[String] and
(__ \ "bar").readNullable[Bar]
)(FooBuilder.apply _)
val writer: Writes[Foo] = (
(__ \ "id").write[Int] and
(__ \ "value").write[String] and
(__ \ "status").write[String] and
(__ \ "bar").write[Bar]
)(unlift(Foo.unapply))
}
This probably won't satisfy the "least possible noise" requirement, but why not introduce the new parameter as an Option[String]?
case class Foo(id:String,value:String, status:Option[String] = Some("pending"))
When reading a Foo from an old client, you'll get a None, which I'd then handle (with a getOrElse) in your consumer code.
Or, if you don't like this, introduce an BackwardsCompatibleFoo:
case class BackwardsCompatibleFoo(id:String,value:String, status:Option[String] = "pending")
case class Foo(id:String,value:String, status: String = "pending")
and then turn that one into a Foo to work with further on, avoiding to have to deal with this kind of data gymnastics all along in the code.
You may define status as an Option
case class Foo(id:String, value:String, status: Option[String])
use JsPath like so:
(JsPath \ "gender").readNullable[String]