graphql-patterns

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• SKILL.md

SKILL.md (first lines)

# GraphQL Patterns

Production-grade GraphQL API design with performance and type safety.

## Schema Design Principles

```graphql
# Use interfaces for shared fields
interface Node {
  id: ID!
  createdAt: DateTime!
  updatedAt: DateTime!
}

type User implements Node {
  id: ID!
  createdAt: DateTime!
  updatedAt: DateTime!
  email: String!
  displayName: String!
  posts(first: Int, after: String): PostConnection!
}

# Relay-style pagination (cursor-based)
type PostConnection {
  edges: [PostEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type PostEdge {
  node: Post!
  cursor: String!
}

type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}

# Input types for mutations
input CreatePostInput {
  title: String!
  body: String!
  tags: [String!]
}

# Union for mutation results (error handling without exceptions)
type CreatePostSuccess {
  post: Post!
}

type ValidationError {
  field: String!
  message: String!
}

union CreatePostResult = CreatePostSuccess | ValidationError
```

## DataLoader - N+1 Prevention

```typescript
import DataLoader from 'dataloader'

// Batch function: receives array of keys, returns array of results in same order
function createUserLoader(db: Database) {
  return new DataLoader<string, User | null>(async (userIds) => {
    const users = await db.user.findMany({
      where: { id: { in: [...userIds] } }
    })
    const userMap = new Map(users.map(u => [u.id, u]))
    // MUST return in same order as input keys
    return userIds.map(id => userMap.get(id) ?? null)
  })
}

// Create per-request context (loaders are NOT shared across requests)
function createContext(req: Request) {
  const db = getDatabase()
  return {
    db,
    loaders: {
      user: createUserLoader(db),
      post: createPostLoader(db),
      comment: createCommentLoader(db),
    }
  }
}

// Resolver uses loader instead of direct DB query
const resolvers = {
  Post: {
    author: (post: Post, _args: unknown, ctx: Context) => {
      return ctx.loaders.user.load(post.authorId)  // batched automatically
    }
  }
}
```

## Resolver Pattern with Validation

```typescript
import { z } from 'zod'

const CreatePostSchema = z.object({
  title: z.string().min(1).max(200),
  body: z.string().min(10).max(50000),
  tags: z.array(z.string()).max(10).optional()
})

const resolvers = {
  Mutation: {
    createPost: async (_parent: unknown, args: { input: unknown }, ctx: Context) => {
      // Auth guard
      if (!ctx.currentUser) {
        throw new AuthenticationError('Login required')

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