Fix aliased shared root fields

.changeset/tricky-jars-wink.md

@@ -0,0 +1,6 @@
+---
+'@graphql-tools/delegate': minor
+'@graphql-tools/federation': patch
+---
+
+Handle shared root field queries with aliases

packages/delegate/src/index.ts

@@ -10,3 +10,4 @@ export * from './subschemaConfig.js';
 export * from './types.js';
 export * from './extractUnavailableFields.js';
 export * from './leftOver.js';
+export * from './symbols.js';

packages/federation/src/supergraph.ts

@@ -36,11 +36,13 @@
   BatchingOptions,
   delegateToSchema,
   extractUnavailableFieldsFromSelectionSet,
+  isExternalObject,
   MergedFieldConfig,
   MergedTypeConfig,
   SubschemaConfig,
   subtractSelectionSets,
   Transform,
+  UNPATHED_ERRORS_SYMBOL,
 } from '@graphql-tools/delegate';
 import { buildHTTPExecutor, HTTPExecutorOptions } from '@graphql-tools/executor-http';
 import {
@@ -1320,6 +1322,13 @@
 
 const specifiedTypeNames = ['ID', 'String', 'Float', 'Int', 'Boolean', '_Any', '_Entity'];
 
+function makeExternalObject(data: unknown, errors: Error[]) {
+  if (!isExternalObject(data) && typeof data === 'object' && data != null) {
+    data[UNPATHED_ERRORS_SYMBOL] = [];
+  }
+  return data;
+}
+
 function mergeResults(results: unknown[]) {
   const errors: Error[] = [];
   const datas: unknown[] = [];
@@ -1328,14 +1337,20 @@
       errors.push(...result.errors);
     } else if (result instanceof Error) {
       errors.push(result);
-    } else {
+    } else if (result != null) {
       datas.push(result);
     }
   }
   if (datas.length) {
-    return mergeDeep(datas, false, true, true);
+    if (datas.length === 1) {
+      return makeExternalObject(datas[0], errors);
+    }
+    return makeExternalObject(mergeDeep(datas), errors);
   }
   if (errors.length) {
+    if (errors.length === 1) {
+      throw errors[0];
+    }
     return new AggregateError(errors, errors.map(error => error.message).join(', \n'));
   }
   return null;

packages/federation/test/aliased-shared-root.test.ts

@@ -0,0 +1,111 @@
+import { parse } from 'graphql';
+import { buildSubgraphSchema } from '@apollo/subgraph';
+import { normalizedExecutor } from '@graphql-tools/executor';
+import { getStitchedSchemaFromLocalSchemas } from './getStitchedSchemaFromLocalSchemas';
+
+describe('Aliased Shared Root Fields', () => {
+  it('issue #6613', async () => {
+    const query = /* GraphQL */ `
+      query {
+        testNestedField {
+          subgraph1 {
+            id
+            email
+            sub1
+          }
+          testUserAlias: subgraph2 {
+            id
+            email
+            sub2
+          }
+        }
+      }
+    `;
+
+    const expectedResult = {
+      data: {
+        testNestedField: {
+          subgraph1: {
+            id: 'user1',
+            email: '[email protected]',
+            sub1: true,
+          },
+          testUserAlias: {
+            id: 'user2',
+            email: '[email protected]',
+            sub2: true,
+          },
+        },
+      },
+    };
+
+    const subgraph1 = buildSubgraphSchema({
+      typeDefs: parse(/* GraphQL */ `
+        type Query {
+          testNestedField: TestNestedField
+        }
+
+        type TestNestedField {
+          subgraph1: TestUser1!
+        }
+
+        type TestUser1 {
+          id: String!
+          email: String!
+          sub1: Boolean!
+        }
+      `),
+      resolvers: {
+        Query: {
+          testNestedField: () => ({
+            subgraph1: () => ({
+              id: 'user1',
+              email: '[email protected]',
+              sub1: true,
+            }),
+          }),
+        },
+      },
+    });
+    const subgraph2 = buildSubgraphSchema({
+      typeDefs: parse(/* GraphQL */ `
+        type Query {
+          testNestedField: TestNestedField
+        }
+
+        type TestNestedField {
+          subgraph2: TestUser2!
+        }
+
+        type TestUser2 {
+          id: String!
+          email: String!
+          sub2: Boolean!
+        }
+      `),
+      resolvers: {
+        Query: {
+          testNestedField: () => ({
+            subgraph2: () => ({
+              id: 'user2',
+              email: '[email protected]',
+              sub2: true,
+            }),
+          }),
+        },
+      },
+    });
+
+    const gatewaySchema = await getStitchedSchemaFromLocalSchemas({
+      subgraph1,
+      subgraph2,
+    });
+
+    const result = await normalizedExecutor({
+      schema: gatewaySchema,
+      document: parse(query),
+    });
+
+    expect(result).toEqual(expectedResult);
+  });
+});