Discard wrapper data when using Reads[T] - json

Question

I think what I'm trying to do should be relatively simple, but I'm not certain. I'm making an API call to get a list of applications and I'd like to read them out. However, the list is nested into the response. The response looks something like:
{
"response": {
"instances": [
{ /* object I'm concerned with reading */ },
...
]
}
}
I have a reader currently defined as:
implicit val appReader : Reads[App] = (
(__ \ "ip_address").read[String] and
(__ \ "hostname").read[String] and
(__ \ "application_version").read[String]
)(App)
And a class of:
case class App(
ip: String,
hostname: String,
version: String
)
However, I'm not sure of how to get at the data since it is buried in the response or if there is a way that I can discard that data.

Answer 1

I assume that the applications lie within the instances array like this:
val js = Json.parse("""
{
"response": {
"instances": [
{ "ip_address": "192.168.1.1", "hostname": "host1", "application_version": "1.0"},
{ "ip_address": "192.168.1.2", "hostname": "host2", "application_version": "1.0"}
]
}
}
""")
Now, to read the list of applications defined as App:
val responseRead =
(__ \ "response").read(
(__ \ "instances").read[List[App]])
val apps = responseRead.reads(js).get
This gives you the list of applications leaving out the rest. If the data structure is somewhat different than assumed, this should point you in the right direction.
Edit: Most of the time I prefer to use the Json macro inception of play. The code for your example could be written as follows:
case class App(
ip_address: String,
hostname: String,
application_version: String)
object App {
implicit val appFormat = Json.format[App]
}
case class Response(
instances: List[App])
object Response {
implicit val responseFormat = Json.format[Response]
}
case class Root(
response: Response)
object Root {
implicit val rootFormat = Json.format[Root]
}
val root = js.as[Root]
val apps = root.response.instances
As you can see, the case class vals must have the same name as their Json counter parts. Additionally, I used format instead of reads which is of course up to you, but most of the time you need both reads and writes which is what format gives you.