Resolver Development

/**
 * Marks a method as a GraphQL Field Resolver for computed fields
 * @param name - Optional field name (defaults to method name)
 * @param options - Configuration options for the field resolver
 */
function ResolveField(name?: string, options?: FieldOptions): MethodDecorator;

/**
 * Marks a method as a GraphQL Reference Resolver for Apollo Federation
 * Used to resolve entity references across federated services
 */
function ResolveReference(): MethodDecorator;

/**
 * Legacy alias for ResolveField (deprecated in favor of ResolveField)
 * @param name - Optional field name
 * @param options - Configuration options
 */
function ResolveProperty(name?: string, options?: FieldOptions): MethodDecorator;

import { Resolver, ResolveField, ResolveReference, Parent, Args } from "@nestjs/graphql";

@Resolver(() => User)
class UserResolver {
  // Computed field resolver
  @ResolveField(() => String)
  fullName(@Parent() user: User): string {
    return `${user.firstName} ${user.lastName}`;
  }
  
  // Async field resolver with database lookup
  @ResolveField(() => [Post])
  async posts(@Parent() user: User): Promise<Post[]> {
    return this.postService.findByUserId(user.id);
  }
  
  // Field resolver with arguments
  @ResolveField(() => [Post])
  recentPosts(
    @Parent() user: User,
    @Args('limit', { defaultValue: 10 }) limit: number
  ): Promise<Post[]> {
    return this.postService.findRecentByUserId(user.id, limit);
  }
  
  // Federation reference resolver
  @ResolveReference()
  resolveReference(reference: { __typename: string; id: string }): Promise<User> {
    return this.userService.findById(reference.id);
  }
}

/**
 * GraphQL-specific execution context wrapper providing access to GraphQL resolver arguments
 */
export class GqlExecutionContext {
  /**
   * Create GraphQL execution context from standard NestJS execution context
   * @param context - Standard NestJS execution context
   * @returns GraphQL-specific execution context
   */
  static create(context: ExecutionContext): GqlExecutionContext;
  
  /**
   * Get the context type (always returns 'graphql')
   * @returns Context type string
   */
  getType(): string;
  
  /**
   * Get the GraphQL root/parent object
   * @returns Root object for the current resolver
   */
  getRoot<T = any>(): T;
  
  /**
   * Get GraphQL resolver arguments
   * @returns Arguments object passed to the resolver
   */
  getArgs<T = any>(): T;
  
  /**
   * Get GraphQL context object
   * @returns GraphQL context containing request, response, and custom data
   */
  getContext<T = any>(): T;
  
  /**
   * Get GraphQL resolve info object
   * @returns GraphQL resolve info containing field selection and metadata
   */
  getInfo(): GraphQLResolveInfo;
}

/**
 * Type alias for GraphQL execution context
 */
type GraphQLExecutionContext = GqlExecutionContext;

/**
 * Union type for GraphQL context types
 */
type GqlContextType = 'graphql';

import { Injectable, CanActivate, ExecutionContext } from "@nestjs/common";
import { GqlExecutionContext } from "@nestjs/graphql";

// Using in a Guard
@Injectable()
export class AuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const gqlContext = GqlExecutionContext.create(context);
    const ctx = gqlContext.getContext();
    const user = ctx.req.user;
    return !!user;
  }
}

// Using in an Interceptor
@Injectable()
export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    const gqlContext = GqlExecutionContext.create(context);
    const info = gqlContext.getInfo();
    console.log(`Executing GraphQL field: ${info.fieldName}`);
    return next.handle();
  }
}

/**
 * Host service for accessing GraphQL resolver arguments
 */
export class GqlArgumentsHost {
  /**
   * Get the GraphQL root/parent object
   * @returns Root object for the current resolver
   */
  getRoot<T = any>(): T;
  
  /**
   * Get GraphQL resolver arguments
   * @returns Arguments object passed to the resolver
   */
  getArgs<T = any>(): T;
  
  /**
   * Get GraphQL context object
   * @returns GraphQL context containing request, response, and custom data
   */
  getContext<T = any>(): T;
  
  /**
   * Get GraphQL resolve info object
   * @returns GraphQL resolve info containing field selection and metadata
   */
  getInfo(): GraphQLResolveInfo;
}

/**
 * Interface for GraphQL arguments host
 */
interface GraphQLArgumentsHost {
  getRoot<T = any>(): T;
  getArgs<T = any>(): T;
  getContext<T = any>(): T;
  getInfo(): GraphQLResolveInfo;
}

/**
 * Base class for services that explore NestJS modules for GraphQL metadata
 */
export abstract class BaseExplorerService {
  /**
   * Explore modules and extract metadata
   * @param modules - Array of NestJS modules to explore
   * @returns Extracted metadata
   */
  protected abstract explore(modules: any[]): any;
  
  /**
   * Filter and process discovered providers
   * @param providers - Array of providers to process
   * @returns Processed providers
   */
  protected abstract filterProviders(providers: any[]): any[];
}

/**
 * Interface for GraphQL context containing request and response objects
 */
interface GraphQLContext {
  /** HTTP request object */
  req: any;
  /** HTTP response object */
  res: any;
  /** Connection object for subscriptions */
  connection?: any;
  /** Additional context data */
  [key: string]: any;
}

/**
 * Type for resolver function signatures
 */
type ResolverFn<TSource = any, TContext = any, TArgs = any, TResult = any> = (
  source: TSource,
  args: TArgs,
  context: TContext,
  info: GraphQLResolveInfo
) => TResult | Promise<TResult>;

/**
 * Type for field resolver functions
 */
type FieldResolverFn<TSource = any, TContext = any, TArgs = any, TResult = any> = (
  source: TSource,
  args: TArgs,
  context: TContext,
  info: GraphQLResolveInfo
) => TResult | Promise<TResult>;

/**
 * Interface for implementing resolve type functions for union and interface types
 */
interface ResolveTypeFn<TSource = any, TContext = any> {
  (value: TSource, context: TContext, info: GraphQLResolveInfo): string | Promise<string>;
}

/**
 * Options for configuring resolver validation
 */
interface ResolverValidationOptions {
  /** Whether to require resolvers for all fields */
  requireResolversForResolveType?: boolean;
  /** Whether to require resolvers for all args */
  requireResolversForArgs?: boolean;
  /** Whether to require resolvers for non-scalar fields */
  requireResolversForNonScalar?: boolean;
  /** Whether to require resolvers for all fields */
  requireResolversForAllFields?: boolean;
}

import { Resolver, ResolveField, Parent, Args, Context, Info } from "@nestjs/graphql";
import { GraphQLResolveInfo } from "graphql";

@Resolver(() => User)
export class AdvancedUserResolver {
  constructor(
    private userService: UserService,
    private postService: PostService,
    private dataLoader: DataLoaderService
  ) {}

  // Resolver with data loader for N+1 problem prevention
  @ResolveField(() => [Post])
  async posts(@Parent() user: User): Promise<Post[]> {
    return this.dataLoader.postsByUserId.load(user.id);
  }
  
  // Resolver with field selection optimization
  @ResolveField(() => Profile)
  async profile(
    @Parent() user: User,
    @Info() info: GraphQLResolveInfo
  ): Promise<Profile> {
    const selectedFields = this.getSelectedFields(info);
    return this.userService.findProfileWithFields(user.id, selectedFields);
  }
  
  // Resolver with complex arguments and validation
  @ResolveField(() => [Post])
  async postsByCategory(
    @Parent() user: User,
    @Args('category') category: string,
    @Args('pagination') pagination: PaginationArgs,
    @Context() context: GraphQLContext
  ): Promise<Post[]> {
    // Access user from context for authorization
    const currentUser = context.req.user;
    return this.postService.findByUserAndCategory(
      user.id, 
      category, 
      pagination,
      currentUser
    );
  }
  
  // Federation resolver with error handling
  @ResolveReference()
  async resolveReference(
    reference: { __typename: string; id: string }
  ): Promise<User | null> {
    try {
      return await this.userService.findById(reference.id);
    } catch (error) {
      // Log error and return null for graceful degradation
      console.error(`Failed to resolve user reference: ${reference.id}`, error);
      return null;
    }
  }
  
  private getSelectedFields(info: GraphQLResolveInfo): string[] {
    // Helper method to extract selected fields from GraphQL info
    // Implementation would parse the selection set
    return [];
  }
}

// Union type resolver
@Resolver(() => SearchResult)
export class SearchResultResolver {
  @ResolveField()
  __resolveType(value: any): string {
    if (value instanceof User) return 'User';
    if (value instanceof Post) return 'Post';
    return null;
  }
}

/**
 * GraphQL-specific error types
 */
import { 
  GraphQLError,
  UserInputError,
  AuthenticationError,
  ForbiddenError,
  ApolloError
} from 'apollo-server-express';

/**
 * Custom GraphQL error with extensions
 */
class CustomGraphQLError extends GraphQLError {
  constructor(
    message: string,
    code: string,
    extensions?: Record<string, any>
  ) {
    super(message, null, null, null, null, null, {
      code,
      ...extensions,
    });
  }
}

import { UserInputError, AuthenticationError } from 'apollo-server-express';

@Resolver(() => User)
export class UserResolver {
  @Query(() => User)
  async user(@Args('id') id: string, @Context() context: any): Promise<User> {
    // Authentication check
    if (!context.user) {
      throw new AuthenticationError('You must be logged in to view user details');
    }
    
    // Input validation
    if (!id || typeof id !== 'string') {
      throw new UserInputError('Invalid user ID provided', {
        invalidArgs: ['id'],
      });
    }
    
    const user = await this.userService.findById(id);
    
    if (!user) {
      throw new UserInputError('User not found', {
        code: 'USER_NOT_FOUND',
        userId: id,
      });
    }
    
    return user;
  }
}