GraphQL Mutations

Expert guidance for designing effective GraphQL mutations.

Quick Reference

Pattern	Use When	Structure
Result payload	All mutations	mutationName(input): MutationNamePayload!
Field-specific errors	Validation failures	errors: [FieldError!]! in payload
Input objects	Complex arguments	input: MutationNameInput!
Noun + Verb naming	State changes	createUser, deletePost, closeCard
Idempotent mutations	Safe retries	Design for repeatable calls
Optimistic UI	Client-side updates	Return predicted result

What Do You Need?

1. Payload design - Return types, error handling
2. Input objects - Structuring mutation arguments
3. Error patterns - Field-specific vs top-level errors
4. Naming - Mutation naming conventions
5. Side effects - Handling async operations

Specify a number or describe your mutation scenario.

Routing

Response	Reference to Read
1, "payload", "return", "response"	payloads.md
2, "input", "argument", "parameter"	inputs.md
3, "error", "validation", "field error"	errors.md
4, "naming", "convention"	naming.md
5, general mutations	Read relevant references

Critical Rules

• Always return a payload: Never just a boolean or the object
• Use input objects for complex arguments: Don't use many scalars
• Field-specific errors in response: Let clients handle per-field failures
• Noun + verb naming: createUser, deleteUser, not user
• Mutations are POST-only: Never use GET for mutations
• Design for idempotency: Safe to call multiple times

Mutation Template

# Input object for complex arguments
input CreateUserInput {
    name: String!
    email: String!
    password: String!
}

# Payload with result and errors
type CreateUserPayload {
    user: User
    errors: [UserError!]!
}

# Field-specific error type
type UserError {
    field: [String!]!  # Path to field: ["email"] or ["user", "emails", 0]
    message: String!
}

# Mutation definition
type Mutation {
    """
    Creates a new user account
    """
    createUser(input: CreateUserInput!): CreateUserPayload!
}

Mutation Implementation

// Good: Mutation with proper payload and field errors
func (r *mutationResolver) CreateUser(ctx context.Context, input CreateUserInput) (*CreateUserPayload, error) {
    // Validate
    var errs []UserError
    if input.Name == "" {
        errs = append(errs, UserError{
            Field:   []string{"name"},
            Message: "Name is required",
        })
    }
    if !isValidEmail(input.Email) {
        errs = append(errs, UserError{
            Field:   []string{"email"},
            Message: "Invalid email format",
        })
    }
    if len(errs) > 0 {
        return &CreateUserPayload{Errors: errs}, nil
    }

    // Create
    user, err := r.db.CreateUser(input)
    if err != nil {
        if errors.Is(err, db.ErrDuplicate) {
            return &CreateUserPayload{
                Errors: []UserError{{
                    Field:   []string{"email"},
                    Message: "Email already exists",
                }},
            }, nil
        }
        return nil, fmt.Errorf("failed to create user")
    }

    return &CreateUserPayload{User: user, Errors: []UserError{}}, nil
}

Common Mutation Patterns

Create

type Mutation {
    createUser(input: CreateUserInput!): CreateUserPayload!
}

type CreateUserPayload {
    user: User
    errors: [UserError!]!
}

Update

type Mutation {
    updateUser(id: ID!, input: UpdateUserInput!): UpdateUserPayload!
}

type UpdateUserPayload {
    user: User
    errors: [UserError!]!
}

Delete

type Mutation {
    deleteUser(id: ID!): DeleteUserPayload!
}

type DeleteUserPayload {
    deletedUserId: ID
    errors: [UserError!]!
}

State Change (Noun + Verb)

type Mutation {
    """
    Closes a card (marks as closed, not deleted)
    """
    closeCard(id: ID!): CloseCardPayload!
}

type CloseCardPayload {
    card: Card
    errors: [UserError!]!
}

Error Handling Patterns

Error Type	Response Pattern
Validation errors	Return in payload errors field
Duplicate unique key	Return in payload errors field
Not found	Return in payload errors field
Permission denied	Return in payload errors field
Internal server error	Return nil, wrap error (don't expose)

HTTP Semantics

Concern	Guidance
HTTP method	Always POST for mutations
Caching	Mutations are never cached
Idempotency	Design mutations to be safely repeatable
Side effects	Document non-obvious side effects
Async operations	Return payload with job ID, query for status

Common Mutation Mistakes

Mistake	Severity	Fix
Returning just boolean	Medium	Use payload with result
No field-specific errors	High	Add errors array to payload
Too many scalar arguments	Medium	Use input object
Verb + noun naming	Low	Use noun + verb (createUser)
Using GET for mutations	Critical	Always use POST
No validation errors in payload	High	Return validation failures

Reference Index

File	Topics
payloads.md	Result types, error patterns, response structure
inputs.md	Input objects, nested inputs, validation
errors.md	Field errors, error types, client handling
naming.md	Conventions, verb selection, consistency

Success Criteria

Mutations are well-designed when:

• All mutations return a payload type
• Field-specific errors returned in payload
• Input objects used for complex arguments
• Noun + verb naming (createUser, deletePost)
• POST only (never GET)
• Idempotent where possible
• Validation errors returned, not thrown
• No internal errors exposed to clients