GraphQL Resolvers Explained

# CHAPTER 10

GraphQL Resolvers Explained #

1. Introduction #

2. Learning Objectives #

• Define what a Resolver function is in GraphQL.

• Understand the 4 standard arguments passed to every resolver ($root, $args, $context, $info).

• Write resolver functions in PHP to fetch data from an array or database.

• Explain the execution flow of a GraphQL query on the server.

3. Beginner-Friendly Explanation #

1. 1. Give me a book on Space.

1. 2. Give me the author's name.

The Schema is the rulebook that says the library actually has books on space. The Resolver is the librarian physically walking to the shelf, finding the book, getting the data, and bringing it back to the desk.

Every single field in a GraphQL query has a dedicated Resolver (librarian) associated with it. When you ask for book, the Book Resolver goes to the database. When you ask for author inside the book, the Author Resolver looks at the book, sees the author ID, and goes to find the author's details.

4. Real-World Examples #

• Fetching from MySQL: A resolver for getUser(id: 1) runs a SQL query: SELECT * FROM users WHERE id = 1, and returns the row.

• Fetching from a Third-Party API: A resolver for currentWeather might use cURL to fetch data from the OpenWeatherMap REST API and return the JSON.

• Calculated Fields: A resolver for fullName might take the firstName and lastName from the database, concatenate them, and return the combined string.

5. Detailed Code Examples #

The Scenario: A simple query fetching a user.

query {
  user(id: 1) {
    name
  }
}

The PHP Resolvers:

<?php
// Mock Database
$usersDB = [
    1 => [&#039;name' => 'Alice', 'email' => 'alice@example.com'],
    2 => [&#039;name' => 'Bob', 'email' => 'bob@example.com']
];

$queryType = new ObjectType([
    &#039;name' => 'Query',
    &#039;fields' => [
        &#039;user' => [
            &#039;type' => $userType,
            &#039;args' => [
                &#039;id' => Type::nonNull(Type::int())
            ],
            // THIS IS THE RESOLVER FUNCTION
            &#039;resolve' => function ($root, $args, $context, $info) use ($usersDB) {
                $userId = $args[&#039;id'];
                
                // Fetch data (could be a MySQL PDO query here)
                if (isset($usersDB[$userId])) {
                    return $usersDB[$userId];
                }
                return null;
            }
        ]
    ]
]);
?>

6. The 4 Resolver Arguments #

1. 1. $root (or $parent): The result of the previous resolver. If resolving the author of a post, $root contains the post data.

1. 2. $args: The arguments passed in the query (e.g., id: 1).

1. 3. $context: An object shared across all resolvers. Used for storing the logged-in User's ID, database connections, or authentication tokens.

1. 4. $info: Advanced details about the query's execution state (rarely used by beginners).

7. Resolving Nested Queries #

query {
  post(id: 5) {     # 1. Post Resolver runs
    title           # 2. Default resolver extracts 'title' from Post data
    author {        # 3. Author Resolver runs, using Post data as $root
      name          # 4. Default resolver extracts 'name' from Author data
    }
  }
}

8. Default Resolvers #

9. Best Practices #

• Keep Resolvers Thin: Do not put complex business logic inside the resolver. The resolver should only extract $args, call a dedicated service class (e.g., UserService->getUserById($id)), and return the result.

• Use Context for Dependency Injection: Pass your PDO database connection and authenticated user session into the $context object so all resolvers can access them without using global variables.

10. Common Mistakes #

• The N+1 Problem: This is the most famous GraphQL pitfall. If you query 100 posts, and each post resolves its author, the Author resolver might execute a SQL SELECT 100 times. We will discuss solving this (using DataLoaders) in later chapters.

• Returning the Wrong Type: If the Schema says a field returns an Int, but your resolver returns a string "42", GraphQL will throw an execution error.

11. Mini Exercises #

1. 1. Which resolver argument would you use to get the search term typed by a user?

1. 2. Which resolver argument would you use to check if a user is logged in?

1. 3. What is the $root argument used for?

12. Coding Challenges #

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the $root (or parent) argument in a resolver. Give an example of when it is used.

• Q: What kind of information should be stored in the $context object?

• Q: What happens if a resolver returns null for a field marked as Non-Null (!) in the schema?

15. FAQs #

Q: Are resolvers executed synchronously or asynchronously? A: In Node.js, they are asynchronous (Promises). In standard PHP, execution is synchronous, though libraries like webonyx support asynchronous patterns using Promises for advanced use cases.

16. Summary #

17. Next Chapter Recommendation #