Federation referenceBy broken since v9.2.0: representation parameter undefined

[ericarosalina]

Describe the bug

The auto-generated @ResolveReference() method created by the referenceBy configuration option fails in Apollo Federation when using @nestjs/graphql v13+. The method receives undefined for the representation parameter, causing a crash when trying to access the key field.

This regression was introduced in v9.2.0 with the DataLoader implementation from PR #391 and #396.

Have you read the Contributing Guidelines?

Yes

To Reproduce

1. Create a NestJS project with Apollo Federation v2
2. Configure a basic entity with referenceBy:

// user.entity.ts
import { Directive, Field, ID, ObjectType } from '@nestjs/graphql';

@ObjectType()
@Directive('@key(fields: "id")')
export class User {
  @Field(() => ID)
  id!: string;

  @Field()
  name!: string;
}

// user.module.ts
import { Module } from '@nestjs/common';
import { NestjsQueryGraphQLModule } from '@ptc-org/nestjs-query-graphql';
import { NestjsQueryTypeOrmModule } from '@ptc-org/nestjs-query-typeorm';
import { User } from './user.entity';
import { UserService } from './user.service';

const nestQueryModule = NestjsQueryGraphQLModule.forFeature({
  imports: [NestjsQueryTypeOrmModule.forFeature([User])],
  services: [UserService],
  resolvers: [
    {
      DTOClass: User,
      EntityClass: User,
      ServiceClass: UserService,
      referenceBy: { key: 'id' }, // This generates broken resolver
    },
  ],
});

@Module({
  imports: [nestQueryModule],
  exports: [nestQueryModule],
})
export class UserModule {}

3. Make a federated _entities query
4. Observe crash: Cannot read properties of undefined (reading 'id')

Expected behavior

The auto-generated @ResolveReference() method should correctly receive the representation parameter and resolve the entity.

Root Cause

In v9.2.0, the auto-generated resolveReference method was enhanced with DataLoader support, adding @Context() and @InjectDataLoaderConfig() parameter decorators:

async resolveReference(representation, @Context() context, @InjectDataLoaderConfig() dataLoaderConfig) {
  const id = representation[key]; // Crashes: representation is undefined
  // ...
}

However, @nestjs/graphql's @ResolveReference() decorator has known incompatibilities with parameter decorators (see NestJS issues #945, #2127, #712). When parameter decorators are present, the representation parameter becomes undefined.

Test Coverage Gap

#396 added comprehensive tests for N+1 query prevention, but these tests only cover regular GraphQL queries (e.g., todoLists { items { ... } }). The tests do not exercise the Apollo Federation _entities query or the @ResolveReference() code path that the PR modified.

The PR's integration tests validate DataLoader usage in nested field resolvers (@ResolveField()), but never test the referenceBy auto-generated resolver or make any _entities queries. This is why the regression was not caught - the actual federation entity resolution was never tested.

Reproducible Test Case

Here's a minimal test that reproduces the issue:

// federated-user.spec.ts
import { INestApplication } from '@nestjs/common';
import { Test } from '@nestjs/testing';
import { getApolloServer } from '@nestjs/apollo';
import { ApolloServer } from '@apollo/server';
import { TypedDocumentNode } from '@graphql-typed-document-node/core';
import { gql } from 'graphql-tag';
import { AppModule } from './app.module'; // Your app module

describe('Federated User', () => {
  let app: INestApplication;
  let server: ApolloServer;

  const FederatedUserQuery: TypedDocumentNode<any, any> = gql`
    query FederatedUser($_representations: [_Any!]!) {
      _entities(representations: $_representations) {
        ... on User {
          id
          name
        }
      }
    }
  `;

  beforeAll(async () => {
    const moduleRef = await Test.createTestingModule({
      imports: [AppModule],
    }).compile();

    app = moduleRef.createNestApplication();
    await app.init();
    server = getApolloServer(app);

    // Create a test user in your database
    // (implementation depends on your setup)
  });

  afterAll(async () => {
    await app.close();
  });

  it('should resolve user via _entities query', async () => {
    const response = await server.executeOperation({
      query: FederatedUserQuery,
      variables: {
        _representations: [
          { __typename: 'User', id: 'test-user-id' }
        ],
      },
    });

    // This will fail with: Cannot read properties of undefined (reading 'id')
    expect(response.body.kind).toBe('single');
    if (response.body.kind === 'single') {
      expect(response.body.singleResult.errors).toBeUndefined();
      expect(response.body.singleResult.data?._entities[0]).toMatchObject({
        id: 'test-user-id',
        name: expect.any(String),
      });
    }
  });
});

Desktop

• @ptc-org/nestjs-query-core: 9.2.1
• @ptc-org/nestjs-query-graphql: 9.2.1
• @ptc-org/nestjs-query-typeorm: 9.2.1
• @nestjs/graphql: 13.2.0
• @nestjs/apollo: 13.2.1
• @apollo/subgraph: 2.12.1
• @nestjs/core: 11.1.9
• Node: v24.5.0

Additional context

This worked correctly in v9.1.0 before the DataLoader changes. The issue affects all federated entities using referenceBy with @nestjs/graphql v13+.

This was generated by Claude Code