Go Graphql Resolver

In this tutorial, you'll learn about GraphQL: Resolver Not Called. We cover key concepts, practical examples, and best practices.

GraphQL resolvers -- Implement gqlgen resolvers with correct signatures to handle queries and mutations.

The Problem

gqlgen generates resolver methods that you must implement. Wrong signature, receiver type, or return type causes build errors.

Wrong

func (r *queryResolver) Users(ctx context.Context) ([]User, error) {
    return nil, nil
}

Output:

// Works when generated signature matches.

Right

func (r *queryResolver) Users(ctx context.Context, filter *UserFilter) ([]*User, error) {
    if filter != nil {
        return db.FindUsers(ctx, *filter)
    }
    return db.AllUsers(ctx)
}
func (r *mutationResolver) CreateUser(ctx context.Context, input NewUser) (*User, error) {
    user, err := db.CreateUser(ctx, input)
    if err != nil {
        return nil, fmt.Errorf("create user: %w", err)
    }
    return user, nil
}

Output:

// Resolvers execute GraphQL queries and mutations.

Prevention

• Implement resolver methods on the generated type
• Use ctx for cancellation and tracing
• Return (T, error) for queries
• Resolver type fields are set in NewResolver()
• Use dependency injection for services, DB, etc.

Common Mistakes with graphql resolver

1. Forgetting deriving (Show, Eq) on custom data types needed for debugging
2. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
3. Using head and tail instead of pattern matching, causing runtime errors on empty lists

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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