sangria-graphql: Inline Fragments Not Resolved

Question

Hey, all. I'm using sangria-graphql, and everything works fine... except for inline fragments. I have the following types in the schema:

interface Entity {
  oid: ID!
}
type Dataset implements Entity {
  oid: ID!
  name: String
  ... (other fields)
}
... (other types implementing Entity)
type Issue {
  entity: Entity!
  ... (other fields)
}
type Query {
  validate(many arguments): [Issue!]
  ... (other queries)
}

I send a query like this:

{
  validate(many arguments) {
    entity {
      oid
      ... on Dataset {
        name
      }
    }
}

Even though the oid returned is that of a Dataset instance, no name is returned with it. It's as if the resolver doesn't know that this is an instance of Dataset and treats it only as an instance of Entity.

Some implementation details. The schema is constructed using a Schema.buildFromAst method from a GraphQL document, with the resolveField method implemented:

import sangria.schema._
import sangria.ast.Document
import play.api.libs.json._

// document is an instance of Document
lazy val schema: Schema[Ctx, Any] =
  Schema.buildFromAst(document, new DefaultAstSchemaBuilder[Ctx] {
    override def resolveField(typeDefinition: TypeDefinition,
                              fieldDefinition: FieldDefinition) =
      typeDefinition.name match {
        case "Mutation" => context =>
          fieldDefinition.name match {
            ... // cases for specific mutations
          }
        case "Query" => context =>
          fieldDefinition.name match {
            case "validate" =>
              ... // implementation that returns a Seq[JsValue],
                  // where the Json values are serializations of Issue
            ... // cases for other queries
          }
        case _ => context =>
          ... // resolve the sub-selection fields as described below
      }
  }

The sub-selection fields are resolved as follows:

• if context.value is a JsObject, then its Json field with the name of context.field.name is taken;
• if context.value is a JsString, then it is interpreted as the oid of an Entity, the entity is looked up in the store, using a handle provided by the Ctx context; the entity is retrieved as a JsObject, and its Json field with the name of context.field.name is taken.

As I mentioned, the problem is that inline fragments are not respected. Perhaps, I'm missing something. Perhaps, not just resolveField but something else also needs to be properly implemented. Perhaps, something is wrong with my implementation of resolveField.

What would you suggest? Where, in your opinion, does the problem lie? What would you recommend I do to fix the problem?

Answer 1

Since you are working with a generic JsValue, you need to also override DefaultAstSchemaBuilder.objectTypeInstanceCheck. It should tell the library whether particular JsValue is of specific GraphQL type or not. For example, if you entity JSON has a field like {"type": "Dataset", ...} then you need to check it against the provided ObjectType name.