Validation to prevent all field types as @hidden

lib/src/main/java/graphql/nadel/validation/NadelFieldValidation.kt

@@ -1,6 +1,7 @@
 package graphql.nadel.validation
 
 import graphql.nadel.engine.util.unwrapAll
+import graphql.nadel.schema.NadelDirectives
 import graphql.nadel.validation.NadelSchemaValidationError.IncompatibleArgumentInputType
 import graphql.nadel.validation.NadelSchemaValidationError.IncompatibleFieldOutputType
 import graphql.nadel.validation.NadelSchemaValidationError.MissingArgumentOnUnderlying
@@ -36,6 +37,9 @@ class NadelFieldValidation internal constructor(
         parent: NadelServiceSchemaElement.FieldsContainer,
         overallFields: List<GraphQLFieldDefinition>,
     ): NadelSchemaValidationResult {
+        if(areAllFieldsHidden(overallFields)) {
+            return NadelSchemaValidationError.AllFieldsUsingHiddenDirective(parent)
+        }
         return overallFields
             .asSequence()
             .let { fieldSequence ->
@@ -53,6 +57,10 @@ class NadelFieldValidation internal constructor(
             .toResult()
     }
 
+    private fun areAllFieldsHidden(overallFields: List<GraphQLFieldDefinition>): Boolean {
+        return overallFields.all { it.hasAppliedDirective(NadelDirectives.hiddenDirectiveDefinition.name) };
+    }
+
     context(NadelValidationContext)
     fun validate(
         parent: NadelServiceSchemaElement.FieldsContainer,

lib/src/main/java/graphql/nadel/validation/NadelSchemaValidationError.kt

@@ -409,6 +409,19 @@ sealed interface NadelSchemaValidationError : NadelSchemaValidationResult {
 
         override val subject = overallField
     }
+
+    data class AllFieldsUsingHiddenDirective(
+        val parentType: NadelServiceSchemaElement,
+    ) : NadelSchemaValidationError {
+        val service: Service get() = parentType.service
+
+        override val message = run {
+            val ot = parentType.overall.name
+            "Must have at least one field without @hidden on type $ot"
+        }
+
+        override val subject = parentType.overall
+    }
 }
 
 private fun toString(element: GraphQLNamedSchemaElement): String {

lib/src/test/kotlin/graphql/nadel/validation/NadelFieldValidationTest.kt

@@ -676,5 +676,30 @@ class NadelFieldValidationTest : DescribeSpec({
 
             assert(error.message == "The overall field Query.fieldA defines argument id which does not exist in service test field Query.fieldA")
         }
+
+        it("fails if schema contains all fields as hidden") {
+            val fixture = NadelValidationTestFixture(
+                overallSchema = mapOf(
+                    "test" to """
+                type Query {
+                    echo: String @hidden
+                }
+            """.trimIndent(),
+                ),
+                underlyingSchema = mapOf(
+                    "test" to """
+                type Query {
+                    echo: String
+                }
+            """.trimIndent(),
+                ),
+            )
+
+            val errors = validate(fixture)
+            assert(errors.map { it.message }.isNotEmpty())
+
+            val error = errors.assertSingleOfType<NadelSchemaValidationError>()
+            assert(error.message == "Must have at least one field without @hidden on type Query")
+        }
     }
 })