Maintain original errors where possible

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ### vNEXT
 
+* When concatenating errors maintain a reference to the original for use downstream [Issue #480](https://github.com/apollographql/graphql-tools/issues/480) [PR #637](https://github.com/apollographql/graphql-tools/pull/637)
+
+
 ### v2.21.0
 
 * Make iterall a runtime dependency [PR #627](https://github.com/apollographql/graphql-tools/pull/627)

src/stitching/errors.ts

@@ -77,6 +77,14 @@ export function getErrorsFromParent(
   };
 }
 
+class CombinedError extends Error {
+  public errors: Error[];
+  constructor(message: string, errors: Error[]) {
+    super(message);
+    this.errors = errors;
+  }
+}
+
 export function checkResultAndHandleErrors(
   result: any,
   info: GraphQLResolveInfo,
@@ -94,7 +102,7 @@ export function checkResultAndHandleErrors(
     const newError =
       result.errors.length === 1 && hasResult(result.errors[0])
         ? result.errors[0]
-        : new Error(concatErrors(result.errors));
+        : new CombinedError(concatErrors(result.errors), result.errors);
 
     throw locatedError(
       newError,

src/test/testErrors.ts

@@ -0,0 +1,76 @@
+import { assert } from 'chai';
+import {
+  GraphQLResolveInfo
+} from 'graphql';
+import {
+  checkResultAndHandleErrors
+} from '../stitching/errors';
+
+import 'mocha';
+
+
+class ErrorWithResult extends Error {
+  public result: any;
+  constructor(message: string, result: any) {
+    super(message);
+    this.result = result;
+  }
+}
+
+describe('Errors', () => {
+  describe('checkResultAndHandleErrors', () => {
+    it('persists single error with a result', done => {
+      const result = {
+        errors: [new ErrorWithResult('Test error', 'result')]
+      };
+      try {
+        checkResultAndHandleErrors(result, {} as GraphQLResolveInfo, 'responseKey');
+      } catch (e) {
+        assert.equal(e.message, 'Test error');
+        assert.isUndefined(e.originalError.errors);
+        done();
+      }
+    });
+
+    it('persists original errors without a result', done => {
+      const result = {
+        errors: [new Error('Test error')]
+      };
+      try {
+        checkResultAndHandleErrors(result, {} as GraphQLResolveInfo, 'responseKey');
+      } catch (e) {
+        assert.equal(e.message, 'Test error');
+        assert.isNotEmpty(e.originalError);
+        assert.isNotEmpty(e.originalError.errors);
+        assert.lengthOf(e.originalError.errors, result.errors.length);
+        result.errors.forEach((error, i) => {
+          assert.deepEqual(e.originalError.errors[i], error);
+        });
+
+        done();
+      }
+    });
+
+    it('combines errors and perists the original errors', done => {
+      const result = {
+        errors: [
+          new Error('Error1'),
+          new Error('Error2')
+        ]
+      };
+      try {
+        checkResultAndHandleErrors(result, {} as GraphQLResolveInfo, 'responseKey');
+      } catch (e) {
+        assert.equal(e.message, 'Error1\nError2');
+        assert.isNotEmpty(e.originalError);
+        assert.isNotEmpty(e.originalError.errors);
+        assert.lengthOf(e.originalError.errors, result.errors.length);
+        result.errors.forEach((error, i) => {
+          assert.deepEqual(e.originalError.errors[i], error);
+        });
+
+        done();
+      }
+    });
+  });
+});

src/test/tests.ts

@@ -6,3 +6,4 @@ import './testMocking';
 import './testResolution';
 import './testMakeRemoteExecutableSchema';
 import './testMergeSchemas';
+import './testErrors';