Go Graphql Mutation

In this tutorial, you'll learn about GraphQL: Mutation vs Query. We cover key concepts, practical examples, and best practices.

GraphQL mutations -- Define mutations in schema with proper input types and return types for state-changing operations.

The Problem

Mutations should be defined in type Mutation, not type Query. GraphQL executes mutations sequentially but queries in parallel.

Wrong

type Query {
    createUser(input: NewUser!): User! // Wrong! Mutation in Query type
}

Output:

// Works but violates GraphQL conventions. No sequential execution.

Right

type Mutation {
    createUser(input: NewUser!): User!
    updateUser(id: ID!, input: UpdateUser!): User!
    deleteUser(id: ID!): Boolean!
}
input NewUser {
    name: String!
    email: String!
}
input UpdateUser {
    name: String
    email: String
}

Output:

mutation {
    createUser(input: { name: "Alice", email: "alice@test.com" }) {
        id
        name
    }
}

Prevention

• Define mutations in type Mutation, not Query
• Use input types for mutation arguments
• Return the modified object or status
• Mutations run sequentially (queries run in parallel)
• Validate input and handle errors in resolver

Common Mistakes with graphql mutation

1. Using foldl instead of foldl' causing stack overflow on large lists
2. Forgetting deriving (Show, Eq) on custom data types needed for debugging
3. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable

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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