In our previous article, we covered the concept of schema, where we defined two types - User and Post and established a relationship between them. In this article, we will delve into implementing functions or resolvers that will assist us in resolving this relationship.

What is a resolver?

A resolver is a function that's responsible for populating the data for a single field in your schema.

In GraphQL, a resolver is a set of functions that define how to respond to a request for a particular field in the schema. When a GraphQL query is executed, each field in the query is resolved by a corresponding resolver function.

Key Arguments of a Resolver

A resolver function typically has the following signature:

fieldName: (parent, args, context, info) => data;

1. parent: The object that contains the result returned from the parent resolver

2. args: An object with the arguments passed into the fields in the query

3. context: This is an object shared by all the resolvers in a particular query

4. info: Field-specific information related to the query

Basic Resolver

Let's start implementing the basic resolver with no arguments in it.
In this example, we implement resolvers that will return all posts from the array object.

// we are importing data from a util file
import {users, posts, users_posts} from "../../utils/data"

const resolvers = {
    Query: {
        posts: () => {
            return posts;  // resolver return the posts array
        }
    }
}

export default resolvers;

Output :

Resolver with an argument

We will implement the get user resolver that will get the user based on the ID of the user and return the user and its posts

// args object is used to pass user id i.e. defined in query
getUser: (_parent: any, args: any, _context: any) => {
            if (args?.id) {
                //search the user based on user id in users array
                const searchedUser: any = users.find(user => user?.id == args?.id)
                // search the posts written by the user
                const userPostRelationships = users_posts?.filter(rel => rel?.userId === searchedUser?.id);
                const userPostIds = userPostRelationships.map(rel => rel?.postId);
                // combine the result and return
                searchedUser['posts'] = posts.filter(post => userPostIds.includes(post?.id));
                return searchedUser;
            } else {
                return new Error("ID is required");
            }
        }

Output

Resolver for mutation

We implement a mutation resolver in which we update the post based on the ID provided by the user

Mutation: {
        updatePost: (_parent: any, args: any, _context: any) => {
            if (args?.id) {
                // search the post
                const post: any = posts.find(p => p?.id == args?.id)
                //update the post if post exist
                if (post) {
                    post.title = args?.title ?? post?.title;
                    post.content = args?.content ?? post?.content;
                    //return updated post
                    return post;
                } else {
                    return new Error("Post not found with the provided id");
                }
            } else {
                return new Error("ID is required")
            }
        }
    }

Output

GraphQL resolvers are the workhorses that make your API come alive. Understanding how to craft them efficiently is key to building a performant and scalable GraphQL service. With the examples provided, you now have a foundation to dive deeper into the intricacies of resolver functions and elevate your GraphQL development skills. Happy coding!