Error when returning ListType(<InterfaceType>)

[Chris Toomey]

As distilled in the code below, which is almost as simple as the playground schema, when I try to execute a query that returns a list of InterfaceType (IFType), I get the error

{

  "data": null,

  "errors": [

    {

      "message": "Can't find appropriate subtype for field at path getIFs[0] (line 1, column 3):\n{ getIFs { id } }\n  ^",

      "path": [

        "getIFs",

        0

      ],

      "locations": [

        {

          "line": 1,

          "column": 3

        }

      ]

    }

  ]

}

If I change the query field to return a list of AType it works fine.

I.e., when line 72 executes, I get the above error, but when line 73 executes, I get the expected result.

What do I need to change in order to be able to have my query return IFTypes instead of ATypes?

     1	package domain.graphql
     2	
     3	import sangria.execution.Executor
     4	import sangria.marshalling.json4s.native._
     5	import sangria.parser.QueryParser
     6	import sangria.schema._
     7	
     8	import scala.util.{Failure, Success}
     9	
    10	object ChrisDebug {
    11	
    12	  trait IF {
    13	    def id: String
    14	  }
    15	
    16	  case class A(id: String, num: Int) extends IF
    17	
    18	  case class B(id: String, bool: Boolean) extends IF
    19	
    20	  type IFRepo = Seq[IF]
    21	
    22	  val myIFRepo: IFRepo = Seq(
    23	    A("foo", 12),
    24	    A("bar", 13)
    25	  )
    26	
    27	  val IFType: InterfaceType[Unit, IF] = InterfaceType(
    28	    name = "IFType",
    29	    description = "Base trait",
    30	    fieldsFn = () => fields[Unit, IF](
    31	      Field("id", StringType, Some("id"), resolve = _.value.id)))
    32	
    33	  val AType = ObjectType(
    34	    "AType",
    35	    "A",
    36	    interfaces[Unit, A](IFType),
    37	    fields[Unit, A](
    38	      Field("id", StringType,
    39	        Some("id"),
    40	        resolve = _.value.id),
    41	      Field("num", IntType,
    42	        Some("num"),
    43	        resolve = _.value.num)
    44	    ))
    45	
    46	  val IFQueryType = ObjectType(
    47	    "IFQueryType",
    48	    "IFQueryType",
    49	    fields[IFRepo, Unit](
    50	      Field("getIFs", ListType(IFType),
    51	        resolve = (ctx) => ctx.ctx)
    52	    )
    53	  )
    54	  val IFSchema = Schema(IFQueryType)
    55	
    56	  val AQueryType = ObjectType(
    57	    "AQueryType",
    58	    "AQueryType",
    59	    fields[IFRepo, Unit](
    60	      Field("getIFs", ListType(AType),
    61	        resolve = (ctx) => ctx.ctx.asInstanceOf[Seq[A]])
    62	    )
    63	  )
    64	  val ASchema = Schema(AQueryType)
    65	
    66	  def doQuery() = {
    67	    val query = "{ getIFs { id } }"
    68	    QueryParser.parse(query) match {
    69	      case Success(queryAst) =>
    70	        import scala.concurrent.ExecutionContext.Implicits.global // scalastyle:off
    71	        val res =
    72	          if (true) Executor.execute(IFSchema, queryAst, myIFRepo)
    73	          else Executor.execute(ASchema, queryAst, myIFRepo)
    74	        res
    75	      case Failure(err)      => throw err
    76	    }
    77	  }
    78	
    79	}

I'm using sangria and sangria-json4s-native 1.2.0 and Play 2.4.

thx,

Chris

[Chris Toomey]

FYI, I debugged this and traced the cause to the fact that the IFSchema was not aware of AType, and thus it was unable to resolve that my A objects were instances of AType. I was able to resolve the problem by making IFSchema aware of AType by changing line 54 to 

val IFSchema = Schema(query = IFQueryType, additionalTypes = List(AType))

My IFQueryType only contains a single field that only references the interface type IFType, but I incorrectly assumed the concrete types implementing it would be automatically identified and added via reflection/introspection. Seems like that's a bug (a field referencing an interface is unable without the concrete types implementing it) or at least a strong enhancement candidate. I'll create an issue for it.

[Oleg Ilyenko]

Hi Chris,

Thanks a lot for a detailed description of the issue. Glad that you found that root cause. I think this kind of issue can be compensated with better documentation and error reporting, but it would be quite challenging to fix.

 Given and `IFType` there is no way to know that there is an `AType` if there no explicit reference to it.

One possible solution I'm aware of is to mutate some JVM-global state in a constructor of the `AType` and register it in some kind of registry. Later on, `Schema` can access this registry and discover `AType`. I don't think that it is an appropriate solution for a library though, so I would prefer to avoid introducing it for obvious reasons. If I remember correctly, reference implementation had this solution for a while. But now they have refactored it. So now it looks and works in the same way as in sangria (with explicit `additionalTypes`). Are you aware of any better approach?

As I mentioned, I would suggest improving the documentation and error messages. Do you already have ideas about which parts of the docs or error messages can be improved?

Cheers,

Oleg

[Chris Toomey]

Hi Oleg, I created https://github.com/sangria-graphql/sangria/issues/242 for tracking this. If you'll add your reply to that I'll follow up there.

BTW, thanks for this awesome library and great documentation and examples, it's obvious you've put a ton of work into this.

Chris

[Oleg Ilyenko]

Thanks for kind your words and creating the issue! I would be easier to track the progress on the GitHub.