Test: Missing subscription field should not be an "internal error" (#1106)

A remaining issue with subscriptions is attempting to subscribe to a missing field. The existing implementation creates an internal error which is an improper blame since there is no possibility to fix it internally - it is an issue with the query.

Since there is no clear consistent behavior relative to query/mutation operations (which simply skip over unknown fields), and the corner-case where this occurs should deal with the error - this changes the internal error to an external yielded GraphQL error.

Note: we still require a spec change since there is an ambiguity of this exact scenario.

Author: leebyron

src/subscription/__tests__/subscribe-test.js

@@ -72,7 +72,6 @@ function emailSchemaWithResolvers(subscribeFn, resolveFn) {
           type: EmailEventType,
           resolve: resolveFn,
           subscribe: subscribeFn,
-          // TODO: remove
           args: {
             priority: { type: GraphQLInt },
           },
@@ -332,7 +331,7 @@ describe('Subscription Initialization Phase', () => {
     );
   });
 
-  it('throws an error for unknown root field', async () => {
+  it('resolves to an error for unknown subscription field', async () => {
     const ast = parse(`
       subscription {
         unknownField
@@ -341,10 +340,17 @@ describe('Subscription Initialization Phase', () => {
 
     const pubsub = new EventEmitter();
 
-    expectPromiseToThrow(
-      () => createSubscription(pubsub, emailSchema, ast),
-      'This subscription is not defined by the schema.',
-    );
+    const { subscription } = await createSubscription(pubsub, emailSchema, ast);
+
+    expect(subscription).to.deep.equal({
+      errors: [
+        {
+          message: 'The subscription field "unknownField" is not defined.',
+          locations: [{ line: 3, column: 9 }],
+          path: undefined,
+        },
+      ],
+    });
   });
 
   it('throws an error if subscribe does not return an iterator', async () => {

src/subscription/subscribe.js

@@ -23,7 +23,6 @@ import {
   responsePathAsArray,
 } from '../execution/execute';
 import { GraphQLSchema } from '../type/schema';
-import invariant from '../jsutils/invariant';
 import mapAsyncIterator from './mapAsyncIterator';
 
 import type { ObjMap } from '../jsutils/ObjMap';
@@ -223,8 +222,15 @@ export function createSourceEventStream(
     const responseName = responseNames[0];
     const fieldNodes = fields[responseName];
     const fieldNode = fieldNodes[0];
-    const fieldDef = getFieldDef(schema, type, fieldNode.name.value);
-    invariant(fieldDef, 'This subscription is not defined by the schema.');
+    const fieldName = fieldNode.name.value;
+    const fieldDef = getFieldDef(schema, type, fieldName);
+
+    if (!fieldDef) {
+      throw new GraphQLError(
+        `The subscription field "${fieldName}" is not defined.`,
+        fieldNodes,
+      );
+    }
 
     // Call the `subscribe()` resolver or the default resolver to produce an
     // AsyncIterable yielding raw payloads.