Preview fields using FieldTags in Sangria

preview-fields.scala

class PreviewSchemaFilter extends DefaultAstSchemaBuilder[RequestContext] {
  override def extendObjectType(origin: MatOrigin,
                                existing: ObjectType[RequestContext, _],
                                extensions: List[TypeExtensionDefinition],
                                fields: () => List[Field[RequestContext, Any]],
                                interfaces: List[InterfaceType[RequestContext, Any]],
                                mat: AstSchemaMaterializer[RequestContext]): ObjectType[RequestContext, Any] = {

    existing.asInstanceOf[ObjectType[RequestContext, Any]].copy(fieldsFn = () => fields().filterNot(_.tags.contains(Preview)))
  }
}

object PreviewSchemaFilter {
  //TODO: This can probably be changed in Sangria 1.4
  //Sangria 1.4 allows type extensions and type definitions without fields
  private val emptyStub: Document =
    Document(
      Vector(
        ObjectTypeDefinition(
          name = "GeneratedPreviewSchema",
          description = Some(StringValue("used to generate preview fields")),
          interfaces = Vector.empty,
          fields = Vector(
            FieldDefinition(
              name = "generated",
              fieldType = NamedType("String"),
              arguments = Vector.empty,
              directives = Vector(
                Directive(
                  name = "deprecated",
                  arguments = Vector(
                    Argument("reason", StringValue("used to generate preview fields"))
                  )
                )
              )
            )
          )
        )
      ))

  def filter(sangriaSchema: SangriaSchema): SangriaSchema = {
    AstSchemaMaterializer.extendSchema(sangriaSchema, emptyStub, new PreviewSchemaFilter())
  }
}