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import { ApolloServer } from '@apollo/server';
import { startStandaloneServer } from '@apollo/server/standalone';
import { readFileSync } from 'fs';

// Load schema
const typeDefs = readFileSync('./schema.graphql', 'utf-8');

// Mock data
const users = [
  { id: '1', name: 'Alice', email: 'alice@example.com' },
  { id: '2', name: 'Bob', email: 'bob@example.com' },
];

const posts = [
  { id: '1', title: 'GraphQL Intro', content: 'Learning GraphQL...', authorId: '1' },
  { id: '2', title: 'Apollo Server', content: 'Building APIs...', authorId: '1' },
];

// Resolvers
const resolvers = {
  Query: {
    user: (_, { id }) => users.find(u => u.id === id),
    users: () => users,
    post: (_, { id }) => posts.find(p => p.id === id),
  },

  Mutation: {
    createPost: (_, { title, content, authorId }) => {
      const post = {
        id: String(posts.length + 1),
        title,
        content,
        authorId,
      };
      posts.push(post);
      return post;
    },

    updatePost: (_, { id, title, content }) => {
      const post = posts.find(p => p.id === id);
      if (!post) throw new Error('Post not found');
      if (title) post.title = title;
      if (content) post.content = content;
      return post;
    },

    deletePost: (_, { id }) => {
      const index = posts.findIndex(p => p.id === id);
      if (index === -1) return false;
      posts.splice(index, 1);
      return true;
    },
  },

  User: {
    posts: (user) => posts.filter(p => p.authorId === user.id),
  },

  Post: {
    author: (post) => users.find(u => u.id === post.authorId),
  },
};

// Create server
const server = new ApolloServer({ typeDefs, resolvers });

startStandaloneServer(server, {
  listen: { port: 4000 },
}).then(({ url }) => {
  console.log(`🚀 Server ready at ${url}`);
});

// Using Apollo Client
import { ApolloClient, InMemoryCache, gql } from '@apollo/client';

const client = new ApolloClient({
  uri: 'http://localhost:4000',
  cache: new InMemoryCache(),
});

// Query
const GET_USER = gql`
  query GetUser($id: ID!) {
    user(id: $id) {
      id
      name
      email
      posts {
        id
        title
        publishedAt
      }
    }
  }
`;

const { data } = await client.query({
  query: GET_USER,
  variables: { id: '1' },
});

// Mutation
const CREATE_POST = gql`
  mutation CreatePost($title: String!, $content: String!, $authorId: ID!) {
    createPost(title: $title, content: $content, authorId: $authorId) {
      id
      title
      content
    }
  }
`;

const { data: postData } = await client.mutate({
  mutation: CREATE_POST,
  variables: {
    title: 'New Post',
    content: 'Hello GraphQL!',
    authorId: '1',
  },
});

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type Query {
  # Field with arguments
  user(id: ID!): User
  users(limit: Int = 10, offset: Int = 0): [User!]!

  # Search with multiple arguments
  searchPosts(
    query: String!
    category: String
    limit: Int = 20
  ): [Post!]!
}

type User {
  id: ID!              # Non-null ID scalar
  name: String!        # Non-null String
  email: String!
  age: Int             # Nullable Int
  isActive: Boolean!
  posts: [Post!]!      # Non-null list of non-null Posts
  profile: Profile     # Nullable object type
}

type Profile {
  bio: String
  avatarUrl: String
  website: String
}

type Post {
  id: ID!
  title: String!
  content: String!
  author: User!
  tags: [String!]      # Non-null list, nullable elements
  publishedAt: DateTime
}

input CreateUserInput {
  name: String!
  email: String!
  age: Int
}

input UpdateUserInput {
  name: String
  email: String
  age: Int
}

type Mutation {
  createUser(input: CreateUserInput!): User!
  updateUser(id: ID!, input: UpdateUserInput!): User!
}

interface Node {
  id: ID!
  createdAt: DateTime!
  updatedAt: DateTime!
}

type User implements Node {
  id: ID!
  createdAt: DateTime!
  updatedAt: DateTime!
  name: String!
  email: String!
}

type Post implements Node {
  id: ID!
  createdAt: DateTime!
  updatedAt: DateTime!
  title: String!
  content: String!
}

type Query {
  node(id: ID!): Node  # Can return User or Post
}

union SearchResult = User | Post | Comment

type Query {
  search(query: String!): [SearchResult!]!
}

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enum Role {
  ADMIN
  MODERATOR
  USER
  GUEST
}

enum PostStatus {
  DRAFT
  PUBLISHED
  ARCHIVED
}

type User {
  id: ID!
  name: String!
  role: Role!
}

type Post {
  id: ID!
  title: String!
  status: PostStatus!
}

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// DateTime scalar (TypeScript)
import { GraphQLScalarType, Kind } from 'graphql';

const DateTimeScalar = new GraphQLScalarType({
  name: 'DateTime',
  description: 'ISO 8601 DateTime',

  // Serialize to client (output)
  serialize(value: Date) {
    return value.toISOString();
  },

  // Parse from client (input)
  parseValue(value: string) {
    return new Date(value);
  },

  // Parse from query literal
  parseLiteral(ast) {
    if (ast.kind === Kind.STRING) {
      return new Date(ast.value);
    }
    return null;
  },
});

// Add to resolvers
const resolvers = {
  DateTime: DateTimeScalar,
  // ... other resolvers
};

type User {
  name: String!           # Non-null String
  email: String           # Nullable String

  tags: [String!]!        # Non-null list of non-null Strings
  friends: [User!]        # Nullable list of non-null Users
  posts: [Post]!          # Non-null list of nullable Posts
  comments: [Comment]     # Nullable list of nullable Comments
}

// Define query with variables
const GET_USER = gql`
  query GetUser($id: ID!, $includePosts: Boolean = false) {
    user(id: $id) {
      id
      name
      email
      posts @include(if: $includePosts) {
        id
        title
      }
    }
  }
`;

// Execute with variables
const { data } = await client.query({
  query: GET_USER,
  variables: {
    id: '1',
    includePosts: true,
  },
});

query {
  # Fetch same field with different arguments
  user1: user(id: "1") {
    name
  }
  user2: user(id: "2") {
    name
  }

  # Alias for clarity
  currentUser: me {
    id
    name
  }
}

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type Resolver<TParent, TArgs, TContext, TResult> = (
  parent: TParent,      // Parent object
  args: TArgs,          // Field arguments
  context: TContext,    // Shared context (auth, db, etc.)
  info: GraphQLResolveInfo  // Query metadata
) => TResult | Promise<TResult>;

const resolvers = {
  Query: {
    user: async (_, { id }, { db }) => {
      return db.users.findById(id);
    },

    users: async (_, { limit = 10, offset = 0 }, { db }) => {
      return db.users.findMany({ limit, offset });
    },
  },

  Mutation: {
    createUser: async (_, { input }, { db, userId }) => {
      if (!userId) {
        throw new Error('Authentication required');
      }

      const user = await db.users.create(input);
      return { user, userErrors: [], success: true };
    },
  },

  User: {
    // Field resolver - only called if client requests 'posts'
    posts: async (user, _, { db }) => {
      return db.posts.findByAuthorId(user.id);
    },

    // Computed field
    fullName: (user) => {
      return `${user.firstName} ${user.lastName}`;
    },
  },
};

// ❌ BAD - N+1 queries
const resolvers = {
  Query: {
    users: () => db.users.findMany(), // 1 query
  },
  User: {
    // Called for EACH user - N queries!
    posts: (user) => db.posts.findByAuthorId(user.id),
  },
};

// Querying 100 users = 1 + 100 = 101 database queries!

import DataLoader from 'dataloader';

// Batch function - receives array of keys
async function batchLoadPosts(authorIds: string[]) {
  const posts = await db.posts.findByAuthorIds(authorIds);

  // Group by author ID
  const postsByAuthor = authorIds.map(authorId =>
    posts.filter(post => post.authorId === authorId)
  );

  return postsByAuthor;
}

// Create context with loaders
function createContext({ req }) {
  return {
    db,
    userId: req.userId,
    loaders: {
      posts: new DataLoader(batchLoadPosts),
    },
  };
}

// ✅ GOOD - Batched queries
const resolvers = {
  Query: {
    users: () => db.users.findMany(), // 1 query
  },
  User: {
    // Uses DataLoader - batches all requests into 1 query!
    posts: (user, _, { loaders }) => {
      return loaders.posts.load(user.id);
    },
  },
};

// Querying 100 users = 1 + 1 = 2 database queries!

// DataLoader with caching
const userLoader = new DataLoader(
  async (ids) => {
    const users = await db.users.findByIds(ids);
    return ids.map(id => users.find(u => u.id === id));
  },
  {
    cache: true,           // Enable caching (default)
    maxBatchSize: 100,     // Limit batch size
    batchScheduleFn: (cb) => setTimeout(cb, 10), // Debounce batching
  }
);

// Cache manipulation
userLoader.clear(id);              // Clear single key
userLoader.clearAll();             // Clear entire cache
userLoader.prime(id, user);        // Prime cache with value

import { ApolloServer } from '@apollo/server';
import { expressMiddleware } from '@apollo/server/express4';
import { ApolloServerPluginDrainHttpServer } from '@apollo/server/plugin/drainHttpServer';
import { createServer } from 'http';
import { WebSocketServer } from 'ws';
import { useServer } from 'graphql-ws/lib/use/ws';
import { makeExecutableSchema } from '@graphql-tools/schema';
import express from 'express';

const app = express();
const httpServer = createServer(app);

const schema = makeExecutableSchema({ typeDefs, resolvers });

// WebSocket server for subscriptions
const wsServer = new WebSocketServer({
  server: httpServer,
  path: '/graphql',
});

const serverCleanup = useServer({ schema }, wsServer);

const server = new ApolloServer({
  schema,
  plugins: [
    ApolloServerPluginDrainHttpServer({ httpServer }),
    {
      async serverWillStart() {
        return {
          async drainServer() {
            await serverCleanup.dispose();
          },
        };
      },
    },
  ],
});

await server.start();
app.use('/graphql', express.json(), expressMiddleware(server));

httpServer.listen(4000);

type Subscription {
  postCreated: Post!
  postUpdated(id: ID!): Post!
  messageAdded(channelId: ID!): Message!
  userStatusChanged(userId: ID!): UserStatus!
}

type Message {
  id: ID!
  text: String!
  author: User!
  channelId: ID!
  createdAt: DateTime!
}

enum UserStatus {
  ONLINE
  OFFLINE
  AWAY
}

import { PubSub } from 'graphql-subscriptions';

const pubsub = new PubSub();

const resolvers = {
  Mutation: {
    createPost: async (_, { input }, { db }) => {
      const post = await db.posts.create(input);

      // Publish to subscribers
      pubsub.publish('POST_CREATED', { postCreated: post });

      return { post, success: true, userErrors: [] };
    },

    sendMessage: async (_, { channelId, text }, { db, userId }) => {
      const message = await db.messages.create({
        channelId,
        text,
        authorId: userId,
      });

      pubsub.publish(`MESSAGE_${channelId}`, {
        messageAdded: message,
      });

      return message;
    },
  },

  Subscription: {
    postCreated: {
      subscribe: () => pubsub.asyncIterator(['POST_CREATED']),
    },

    postUpdated: {
      subscribe: (_, { id }) => pubsub.asyncIterator([`POST_UPDATED_${id}`]),
    },

    messageAdded: {
      subscribe: (_, { channelId }) => {
        return pubsub.asyncIterator([`MESSAGE_${channelId}`]);
      },
    },
  },
};

import { ApolloClient, InMemoryCache, split, HttpLink } from '@apollo/client';
import { GraphQLWsLink } from '@apollo/client/link/subscriptions';
import { getMainDefinition } from '@apollo/client/utilities';
import { createClient } from 'graphql-ws';

// HTTP link for queries and mutations
const httpLink = new HttpLink({
  uri: 'http://localhost:4000/graphql',
});

// WebSocket link for subscriptions
const wsLink = new GraphQLWsLink(
  createClient({
    url: 'ws://localhost:4000/graphql',
  })
);

// Split based on operation type
const splitLink = split(
  ({ query }) => {
    const definition = getMainDefinition(query);
    return (
      definition.kind === 'OperationDefinition' &&
      definition.operation === 'subscription'
    );
  },
  wsLink,
  httpLink
);

const client = new ApolloClient({
  link: splitLink,
  cache: new InMemoryCache(),
});

// Use subscription
const MESSAGES_SUBSCRIPTION = gql`
  subscription OnMessageAdded($channelId: ID!) {
    messageAdded(channelId: $channelId) {
      id
      text
      author {
        name
      }
      createdAt
    }
  }
`;

function ChatComponent({ channelId }) {
  const { data, loading } = useSubscription(MESSAGES_SUBSCRIPTION, {
    variables: { channelId },
  });

  if (loading) return <p>Loading...</p>;

  return <div>New message: {data.messageAdded.text}</div>;
}