Graphql API Design

# schema.graphql — Complete blog platform schema

scalar DateTime

enum PostStatus {
  DRAFT
  PUBLISHED
  ARCHIVED
}

type User {
  id: ID!
  username: String!
  email: String!
  bio: String
  avatarUrl: String
  posts(first: Int, after: String): PostConnection!
  createdAt: DateTime!
}

type Post {
  id: ID!
  title: String!
  slug: String!
  content: String!
  excerpt: String
  status: PostStatus!
  author: User!
  tags: [Tag!]!
  comments(first: Int, after: String): CommentConnection!
  publishedAt: DateTime
  createdAt: DateTime!
  updatedAt: DateTime!
}

type Comment {
  id: ID!
  body: String!
  author: User!
  post: Post!
  createdAt: DateTime!
}

type Tag {
  id: ID!
  name: String!
  slug: String!
  posts(first: Int, after: String): PostConnection!
}

# Relay connection types for cursor-based pagination
type PostConnection {
  edges: [PostEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

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

type CommentConnection {
  edges: [CommentEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type CommentEdge {
  cursor: String!
  node: Comment!
}

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

# Queries
type Query {
  post(id: ID, slug: String): Post
  posts(
    first: Int = 10
    after: String
    status: PostStatus
    tagSlug: String
  ): PostConnection!
  user(id: ID!): User
  me: User
  tags: [Tag!]!
}

# Mutations with input types and payload types
input CreatePostInput {
  title: String!
  content: String!
  tagIds: [ID!]
  status: PostStatus = DRAFT
}

type CreatePostPayload {
  post: Post
  errors: [MutationError!]!
}

input UpdatePostInput {
  title: String
  content: String
  status: PostStatus
  tagIds: [ID!]
}

type UpdatePostPayload {
  post: Post
  errors: [MutationError!]!
}

type MutationError {
  field: String
  message: String!
}

type Mutation {
  createPost(input: CreatePostInput!): CreatePostPayload!
  updatePost(id: ID!, input: UpdatePostInput!): UpdatePostPayload!
  deletePost(id: ID!): Boolean!
  addComment(postId: ID!, body: String!): Comment!
}

# Subscriptions
type Subscription {
  commentAdded(postId: ID!): Comment!
  postPublished: Post!
}

// resolvers.js — Resolvers with DataLoader for N+1 prevention
const DataLoader = require("dataloader");

// Create loaders per request (called from context factory)
function createLoaders(db) {
  return {
    userLoader: new DataLoader(async (userIds) => {
      const users = await db.users.findByIds(userIds);
      const userMap = new Map(users.map((u) => [u.id, u]));
      return userIds.map((id) => userMap.get(id) || null);
    }),
    postLoader: new DataLoader(async (postIds) => {
      const posts = await db.posts.findByIds(postIds);
      const postMap = new Map(posts.map((p) => [p.id, p]));
      return postIds.map((id) => postMap.get(id) || null);
    }),
  };
}

const resolvers = {
  Query: {
    post: (_, { id, slug }, { db }) => {
      if (id) return db.posts.findById(id);
      if (slug) return db.posts.findBySlug(slug);
      return null;
    },
    posts: async (_, { first = 10, after, status, tagSlug }, { db }) => {
      const cursor = after ? decodeCursor(after) : null;
      const { rows, totalCount } = await db.posts.findPaginated({
        limit: first + 1,
        cursor,
        status,
        tagSlug,
      });
      const hasNextPage = rows.length > first;
      const edges = rows.slice(0, first).map((post) => ({
        cursor: encodeCursor(post.id),
        node: post,
      }));
      return {
        edges,
        totalCount,
        pageInfo: {
          hasNextPage,
          hasPreviousPage: !!after,
          startCursor: edges[0]?.cursor || null,
          endCursor: edges[edges.length - 1]?.cursor || null,
        },
      };
    },
    me: (_, __, { currentUser }) => currentUser,
  },
  Post: {
    author: (post, _, { loaders }) => loaders.userLoader.load(post.authorId),
    tags: (post, _, { db }) => db.tags.findByPostId(post.id),
  },
  Comment: {
    author: (comment, _, { loaders }) => loaders.userLoader.load(comment.authorId),
  },
  Mutation: {
    createPost: async (_, { input }, { currentUser, db }) => {
      if (!currentUser) return { post: null, errors: [{ message: "Not authenticated" }] };
      if (!input.title.trim()) {
        return { post: null, errors: [{ field: "title", message: "Title cannot be empty" }] };
      }
      const post = await db.posts.create({ ...input, authorId: currentUser.id });
      return { post, errors: [] };
    },
  },
};

function encodeCursor(id) { return Buffer.from(`cursor:${id}`).toString("base64"); }
function decodeCursor(cursor) { return Buffer.from(cursor, "base64").toString().replace("cursor:", ""); }

// pagination.js — Reusable cursor-based pagination for any entity

/**
 * Generic paginated query builder for SQL databases.
 * Works with any table that has an auto-incrementing or sortable ID.
 */
async function paginatedQuery(db, { table, first = 10, after, where = {} }) {
  const limit = Math.min(first, 100); // Cap at 100 per page
  const conditions = [];
  const params = [];

  // Apply cursor (decode to original ID)
  if (after) {
    const cursorId = Buffer.from(after, "base64").toString().split(":")[1];
    conditions.push(`id < $${params.length + 1}`);
    params.push(cursorId);
  }

  // Apply additional filters
  for (const [key, value] of Object.entries(where)) {
    if (value !== undefined) {
      conditions.push(`${key} = $${params.length + 1}`);
      params.push(value);
    }
  }

  const whereClause = conditions.length > 0 ? `WHERE ${conditions.join(" AND ")}` : "";

  // Fetch one extra row to determine hasNextPage
  const query = `SELECT * FROM ${table} ${whereClause} ORDER BY id DESC LIMIT ${limit + 1}`;
  const rows = await db.query(query, params);

  // Count total matching rows
  const countQuery = `SELECT COUNT(*) as total FROM ${table} ${whereClause}`;
  const [{ total: totalCount }] = await db.query(countQuery, params);

  const hasNextPage = rows.length > limit;
  const nodes = rows.slice(0, limit);

  const edges = nodes.map((node) => ({
    cursor: Buffer.from(`cursor:${node.id}`).toString("base64"),
    node,
  }));

  return {
    edges,
    totalCount,
    pageInfo: {
      hasNextPage,
      hasPreviousPage: !!after,
      startCursor: edges[0]?.cursor || null,
      endCursor: edges[edges.length - 1]?.cursor || null,
    },
  };
}

// Usage in resolver
const resolvers = {
  Query: {
    posts: (_, args, { db }) =>
      paginatedQuery(db, {
        table: "posts",
        first: args.first,
        after: args.after,
        where: { status: args.status },
      }),
  },
};