GraphQL Resolvers Skill

Build performant data fetching with proper patterns

Overview

Master resolver implementation including the critical DataLoader pattern for preventing N+1 queries, context design, and error handling strategies.

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Quick Reference

Pattern	Purpose	When to Use
DataLoader	Batch + cache	Any relationship field
Context	Request-scoped data	Auth, loaders, datasources
Field resolver	Computed fields	Derived data
Root resolver	Entry points	Query/Mutation fields

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Core Patterns

1. Resolver Signature

// (parent, args, context, info) => result

const resolvers = {
  Query: {
    // Root resolver - parent is undefined
    user: async (_, { id }, { dataSources }) => {
      return dataSources.users.findById(id);
    },
  },

  User: {
    // Field resolver - parent is User object
    posts: async (user, { first = 10 }, { loaders }) => {
      return loaders.postsByAuthor.load(user.id);
    },

    // Computed field - sync is fine
    fullName: (user) => `${user.firstName} ${user.lastName}`,

    // Default resolver (implicit)
    // email: (user) => user.email,
  },
};

2. DataLoader Pattern

const DataLoader = require('dataloader');

// N+1 Problem:
// Query: { users { posts { title } } }
// Without DataLoader: 1 + N queries

// Solution: Batch loading
const createLoaders = () => ({
  // Batch by foreign key
  postsByAuthor: new DataLoader(async (authorIds) => {
    // 1. Single query for all authors
    const posts = await db.posts.findAll({
      where: { authorId: { [Op.in]: authorIds } }
    });

    // 2. Group by author
    const postsByAuthor = {};
    posts.forEach(post => {
      if (!postsByAuthor[post.authorId]) {
        postsByAuthor[post.authorId] = [];
      }
      postsByAuthor[post.authorId].push(post);
    });

    // 3. Return in same order as input
    return authorIds.map(id => postsByAuthor[id] || []);
  }),

  // Batch by primary key
  userById: new DataLoader(async (ids) => {
    const users = await db.users.findAll({
      where: { id: { [Op.in]: ids } }
    });
    const userMap = new Map(users.map(u => [u.id, u]));
    return ids.map(id => userMap.get(id) || null);
  }),
});

// Usage in resolvers
const resolvers = {
  Post: {
    author: (post, _, { loaders }) => {
      return loaders.userById.load(post.authorId);
    },
  },
  User: {
    posts: (user, _, { loaders }) => {
      return loaders.postsByAuthor.load(user.id);
    },
  },
};

3. Context Setup

const createContext = async ({ req, res }) => {
  // 1. Parse auth token
  const token = req.headers.authorization?.replace('Bearer ', '');
  const user = token ? await verifyToken(token) : null;

  // 2. Create request-scoped loaders (IMPORTANT!)
  const loaders = createLoaders();

  // 3. Initialize data sources
  const dataSources = {
    users: new UserDataSource(db),
    posts: new PostDataSource(db),
  };

  // 4. Request metadata
  const requestId = req.headers['x-request-id'] || crypto.randomUUID();

  return {
    user,
    loaders,
    dataSources,
    requestId,
  };
};

// Apollo Server setup
const server = new ApolloServer({
  typeDefs,
  resolvers,
  context: createContext,
});

4. Error Handling

import { GraphQLError } from 'graphql';

const resolvers = {
  Mutation: {
    createUser: async (_, { input }, { dataSources, user }) => {
      // 1. Auth check
      if (!user) {
        throw new GraphQLError('Not authenticated', {
          extensions: { code: 'UNAUTHENTICATED' }
        });
      }

      // 2. Validation (return errors, don't throw)
      const validationErrors = validateInput(input);
      if (validationErrors.length > 0) {
        return { user: null, errors: validationErrors };
      }

      // 3. Business logic
      try {
        const newUser = await dataSources.users.create(input);
        return { user: newUser, errors: [] };
      } catch (error) {
        // Known error
        if (error.code === 'DUPLICATE_EMAIL') {
          return {
            user: null,
            errors: [{ field: 'email', message: 'Already exists' }]
          };
        }
        // Unknown error - throw
        throw new GraphQLError('Internal error', {
          extensions: { code: 'INTERNAL_ERROR' }
        });
      }
    },
  },
};

5. Subscription Resolvers

import { PubSub, withFilter } from 'graphql-subscriptions';

const pubsub = new PubSub();

const resolvers = {
  Mutation: {
    sendMessage: async (_, { input }, { dataSources }) => {
      const message = await dataSources.messages.create(input);

      // Publish event
      pubsub.publish('MESSAGE_SENT', {
        messageSent: message,
        channelId: input.channelId,
      });

      return message;
    },
  },

  Subscription: {
    // Simple subscription
    userCreated: {
      subscribe: () => pubsub.asyncIterator(['USER_CREATED']),
    },

    // Filtered subscription
    messageSent: {
      subscribe: withFilter(
        () => pubsub.asyncIterator(['MESSAGE_SENT']),
        (payload, variables) => {
          return payload.channelId === variables.channelId;
        }
      ),
    },
  },
};

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Performance Targets

Resolver Type	Target	Action if Exceeded
Simple field	< 10ms	Check DB indexes
DataLoader batch	< 50ms	Optimize query
Complex computation	< 200ms	Consider caching
Total request	< 500ms	Profile and optimize

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Troubleshooting

Issue	Symptom	Solution
N+1 queries	Slow, many DB calls	Use DataLoader
Memory leak	Growing memory	Create loaders per request
Stale data	Wrong results	Clear DataLoader cache
Race condition	Intermittent errors	Don't mutate context

Debug Techniques

// 1. Log DataLoader batches
const loader = new DataLoader(async (keys) => {
  console.log(`Batching ${keys.length} keys`);
  // ...
});

// 2. Time resolvers
const withTiming = (resolver) => async (...args) => {
  const start = Date.now();
  const result = await resolver(...args);
  console.log(`Took ${Date.now() - start}ms`);
  return result;
};

// 3. Request logging plugin
const loggingPlugin = {
  requestDidStart() {
    const start = Date.now();
    return {
      willSendResponse() {
        console.log(`Request took ${Date.now() - start}ms`);
      },
    };
  },
};

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Usage

Skill("graphql-resolvers")

Related Skills

• graphql-schema-design - Schema that resolvers implement
• graphql-apollo-server - Server configuration
• graphql-security - Auth in resolvers

Related Agent

• 03-graphql-resolvers - For detailed guidance