GRAPHQL Mutations

GraphQL Mutations

Expert guidance for designing effective GraphQL mutations.

Quick Reference

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

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

HTTP Semantics

Common Mutation Mistakes

Reference Index

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

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