fix(stitch): handle @oneOfs in subschemas

Author: ardatan

.changeset/rare-chairs-itch.md

@@ -0,0 +1,5 @@
+---
+'@graphql-tools/stitch': patch
+---
+
+Handle `@oneOf` directive properly while stitching subschemas

packages/stitch/src/mergeCandidates.ts

@@ -64,6 +64,9 @@ export function mergeCandidates<TContext = Record<string, any>>(
   typeMergingOptions?: TypeMergingOptions<TContext>,
 ): GraphQLNamedType {
   const initialCandidateType = candidates[0]?.type;
+  if (candidates.length === 1) {
+    return initialCandidateType!;
+  }
   if (
     candidates.some(
       (candidate) =>
@@ -165,6 +168,8 @@ function mergeObjectTypeCandidates<TContext = Record<string, any>>(
     astNode,
     extensionASTNodes,
     extensions,
+    // @ts-expect-error We know isOneOf exists here
+    isOneOf: candidates.every((candidate) => candidate.type.isOneOf),
   };
 
   return new GraphQLObjectType(typeConfig);
@@ -220,6 +225,8 @@ function mergeInputObjectTypeCandidates<TContext = Record<string, any>>(
     astNode,
     extensionASTNodes,
     extensions,
+    // @ts-expect-error We know isOneOf exists here
+    isOneOf: candidates.every((candidate) => candidate.type.isOneOf),
   };
 
   return new GraphQLInputObjectType(typeConfig);

packages/stitch/tests/mergeOneOfs.test.ts

@@ -0,0 +1,27 @@
+import { stitchSchemas } from '@graphql-tools/stitch';
+import { buildSchema, printSchema } from 'graphql';
+import { expect, it } from 'vitest';
+
+it('should merge @oneOf directives', () => {
+  const subschemaSdl = /* GraphQL */ `
+type Query {
+  findPerson(in: FindPersonInput): Person
+  findPersonById(id: String): Person @deprecated(reason: "use findPerson")
+}
+
+type Person {
+  Name: String
+  ID: String
+}
+
+input FindPersonInput @oneOf {
+  byId: String
+  byName: String
+}
+`.trim();
+  const subschema = buildSchema(subschemaSdl);
+  const gatewaySchema = stitchSchemas({
+    subschemas: [{ schema: subschema }],
+  });
+  expect(printSchema(gatewaySchema)).toBe(subschemaSdl);
+});