1. Introduction

In this tutorial, we will learn how to create Query and Mutation resolvers, which are fundamental components of a GraphQL server.

You will learn:
- The roles of Query and Mutation resolvers in a GraphQL API
- How to create Query resolvers to fetch data
- How to create Mutation resolvers to modify data

Prerequisites:
- Basic knowledge of JavaScript
- Familiarity with GraphQL and its syntax

2. Step-by-Step Guide

Resolvers in GraphQL are functions that resolve data for your GraphQL fields in the schema. They're responsible for the process of combining your schema, your query, and your data source information to return the requested data.

Queries

Query resolvers fetch data from your database and return it to your GraphQL server. They are responsible for reading data.

Mutations

Mutation resolvers, on the other hand, are responsible for creating, updating, and deleting data.

Best Practices

• Always return promises in your resolvers, GraphQL can handle them appropriately.
• Keep your resolvers simple, they should do one thing and do it well.

3. Code Examples

Here's an example of a Query resolver and a Mutation resolver for a simple user system.

Query Resolver

// The users Query resolver
const resolvers = {
  Query: {
    users: async () => {
      // fetch all users from the database
      const users = await User.find();
      // return the users
      return users;
    },
  },
};

This code defines a Query resolver for fetching all users. The resolver is an async function that fetches all users from the database using the User.find() function and then returns them.

Mutation Resolver

// The createUser Mutation resolver
const resolvers = {
  Mutation: {
    createUser: async (parent, args) => {
      // create a new user with the provided input
      const newUser = new User(args);
      // save the user to the database
      await newUser.save();
      // return the new user
      return newUser;
    },
  },
};

This code defines a Mutation resolver for creating a new user. The resolver is an async function that creates a new user with the provided arguments, saves the user to the database using the newUser.save() function, and then returns the new user.

4. Summary

In this tutorial, we covered the basics of creating Query and Mutation resolvers in GraphQL. We learned that Query resolvers are for fetching data and Mutation resolvers are for modifying data.

To learn more, you can look into how to handle errors in your resolvers, how to use context in your resolvers, and how to create more complex resolvers.

5. Practice Exercises

1. Create a Query resolver that fetches a single user by their ID.
2. Create a Mutation resolver that updates a user's data.
3. Create a Mutation resolver that deletes a user.

Solutions:

1. Query resolver for a user:

const resolvers = {
  Query: {
    user: async (parent, args) => {
      const user = await User.findById(args.id);
      return user;
    },
  },
};

1. Mutation resolver for updating a user:

const resolvers = {
  Mutation: {
    updateUser: async (parent, args) => {
      const updatedUser = await User.findByIdAndUpdate(args.id, args, { new: true });
      return updatedUser;
    },
  },
};

1. Mutation resolver for deleting a user:

const resolvers = {
  Mutation: {
    deleteUser: async (parent, args) => {
      const deletedUser = await User.findByIdAndDelete(args.id);
      return deletedUser;
    },
  },
};

Each of these exercises will help you understand how to work with resolvers better and make you more comfortable with them. Happy coding!