fix(schema-first): merge duplicate IQuery interface

packages/apollo/tests/e2e/generated-definitions.spec.ts

@@ -339,4 +339,23 @@ describe('Generated Definitions', () => {
   afterEach(async () => {
     await app.close();
   });
+
+  it('should generate for a federated graph with partial query definition', async () => {
+    const outputFile = generatedDefinitions(
+      'federation-partial-query.test-definitions.ts',
+    );
+    const factory = new GraphQLFederationDefinitionsFactory();
+    await factory.generate({
+      typePaths: [generatedDefinitions('federation-partial-query.graphql')],
+      path: outputFile,
+      outputAs: 'interface',
+    });
+
+    expect(
+      await readFile(
+        generatedDefinitions('federation-partial-query.fixture.ts'),
+        'utf8',
+      ),
+    ).toBe(await readFile(outputFile, 'utf8'));
+  });
 });

packages/apollo/tests/generated-definitions/federation-partial-query.fixture.ts

@@ -0,0 +1,15 @@
+
+/*
+ * -------------------------------------------------------
+ * THIS FILE WAS AUTOMATICALLY GENERATED (DO NOT MODIFY)
+ * -------------------------------------------------------
+ */
+
+/* tslint:disable */
+/* eslint-disable */
+
+export interface IQuery {
+    foo(): Nullable<boolean> | Promise<Nullable<boolean>>;
+}
+
+type Nullable<T> = T | null;

packages/apollo/tests/generated-definitions/federation-partial-query.graphql

@@ -0,0 +1,3 @@
+extend type Query {
+    foo: Boolean
+}

packages/graphql/lib/federation/graphql-federation-definitions.factory.ts

@@ -3,6 +3,7 @@ import { gql } from 'graphql-tag';
 import { DefinitionsGeneratorOptions } from '../graphql-ast.explorer';
 import { GraphQLDefinitionsFactory } from '../graphql-definitions.factory';
 import { extend } from '../utils';
+import { mergeTypeDefs } from '@graphql-tools/merge';
 
 export class GraphQLFederationDefinitionsFactory extends GraphQLDefinitionsFactory {
   protected async exploreAndEmit(
@@ -35,9 +36,17 @@ export class GraphQLFederationDefinitionsFactory extends GraphQLDefinitionsFacto
         resolvers: {},
       },
     ]);
+
+    const mergedDefinition = mergeTypeDefs([printSubgraphSchema(schema)], {
+      useSchemaDefinition: false,
+      throwOnConflict: true,
+      commentDescriptions: true,
+      reverseDirectives: true,
+    });
+
     const tsFile = await this.gqlAstExplorer.explore(
       gql`
-        ${printSubgraphSchema(schema)}
+        ${mergedDefinition}
       `,
       path,
       outputAs,