How to Build a User Follow System with NENE2

This guide walks through building a Twitter/Instagram-style social follow system — users follow each other, view follower/following lists, and check mutual follow status.

Field Trial: FT134
NENE2 version: ^1.5
Covered topics: follow/unfollow, idempotent writes, graph queries, soft-constraint enforcement

What we're building

A REST API that lets users:

Follow and unfollow other users (idempotent follow, 204 unfollow)
Query follower/following lists (ordered by most-recent-first)
Check follower/following counts in one stat call
Check if a specific user is following another

Database schema

sql

CREATE TABLE users (
    id         INTEGER PRIMARY KEY AUTOINCREMENT,
    name       TEXT NOT NULL,
    created_at TEXT NOT NULL
);

CREATE TABLE follows (
    id          INTEGER PRIMARY KEY AUTOINCREMENT,
    follower_id INTEGER NOT NULL,
    followee_id INTEGER NOT NULL,
    created_at  TEXT    NOT NULL,
    UNIQUE (follower_id, followee_id),
    CHECK  (follower_id != followee_id),
    FOREIGN KEY (follower_id) REFERENCES users(id),
    FOREIGN KEY (followee_id) REFERENCES users(id)
);

Two constraints do the heavy lifting at the DB layer:

UNIQUE (follower_id, followee_id) — prevents duplicate follow rows
CHECK (follower_id != followee_id) — prevents self-follow at the DB level (application also validates this)

API endpoints

Method	Path	Description
`POST`	`/users`	Create a user
`POST`	`/users/{followerId}/follow`	Follow a user (idempotent)
`DELETE`	`/users/{followerId}/follow/{followeeId}`	Unfollow a user
`GET`	`/users/{userId}/stats`	Get follower/following counts
`GET`	`/users/{userId}/followers`	List followers
`GET`	`/users/{userId}/following`	List users this user follows
`GET`	`/users/{followerId}/is-following/{followeeId}`	Check follow relationship

Repository

php

final class FollowRepository
{
    public function __construct(
        private readonly DatabaseQueryExecutorInterface $executor,
    ) {}

    public function follow(int $followerId, int $followeeId, string $now): bool
    {
        if ($this->isFollowing($followerId, $followeeId)) {
            return false; // already following — return false so handler sends 200
        }

        $this->executor->execute(
            'INSERT INTO follows (follower_id, followee_id, created_at) VALUES (?, ?, ?)',
            [$followerId, $followeeId, $now],
        );

        return true; // new follow — handler sends 201
    }

    public function unfollow(int $followerId, int $followeeId): bool
    {
        $count = $this->executor->execute(
            'DELETE FROM follows WHERE follower_id = ? AND followee_id = ?',
            [$followerId, $followeeId],
        );

        return $count > 0;
    }

    public function isFollowing(int $followerId, int $followeeId): bool
    {
        return $this->executor->fetchOne(
            'SELECT id FROM follows WHERE follower_id = ? AND followee_id = ?',
            [$followerId, $followeeId],
        ) !== null;
    }

    /** @return array<int, array{id: int, name: string}> */
    public function listFollowers(int $userId): array
    {
        $rows = $this->executor->fetchAll(
            'SELECT u.id, u.name FROM users u
             INNER JOIN follows f ON f.follower_id = u.id
             WHERE f.followee_id = ?
             ORDER BY f.id DESC',
            [$userId],
        );

        return array_map(fn(mixed $row) => $this->hydrateUser((array) $row), $rows);
    }

    /** @return array<int, array{id: int, name: string}> */
    public function listFollowing(int $userId): array
    {
        $rows = $this->executor->fetchAll(
            'SELECT u.id, u.name FROM users u
             INNER JOIN follows f ON f.followee_id = u.id
             WHERE f.follower_id = ?
             ORDER BY f.id DESC',
            [$userId],
        );

        return array_map(fn(mixed $row) => $this->hydrateUser((array) $row), $rows);
    }
}

The follow() method returns bool to signal new-vs-existing so the handler can emit the correct HTTP status without an extra query.

Follow handler — idempotent POST

The tricky part is returning 201 Created for a new follow and 200 OK for a repeat follow, while rejecting self-follow with 422.

php

private function followUser(ServerRequestInterface $request): ResponseInterface
{
    $params     = (array) $request->getAttribute(Router::PARAMETERS_ATTRIBUTE);
    $followerId = isset($params['followerId']) && is_numeric($params['followerId'])
        ? (int) $params['followerId'] : 0;

    if ($followerId <= 0 || !$this->repo->findUserById($followerId)) {
        return $this->responseFactory->create(['error' => 'follower not found'], 404);
    }

    $body       = JsonRequestBodyParser::parse($request);
    $followeeId = isset($body['followee_id']) && is_int($body['followee_id'])
        ? $body['followee_id'] : 0;

    if ($followeeId <= 0 || !$this->repo->findUserById($followeeId)) {
        return $this->responseFactory->create(['error' => 'followee not found'], 404);
    }

    if ($followerId === $followeeId) {
        return $this->responseFactory->create(['error' => 'cannot follow yourself'], 422);
    }

    $wasNew = $this->repo->follow($followerId, $followeeId, date('Y-m-d H:i:s'));
    $status = $wasNew ? 201 : 200;

    return $this->responseFactory->create([
        'follower_id' => $followerId,
        'followee_id' => $followeeId,
        'following'   => true,
    ], $status);
}

Why validate before the self-follow check? The 404 checks come first so that POST /users/9999/follow with followee_id: 9999 returns 404 (user doesn't exist) rather than 422 (self-follow). Existence errors take priority.

Unfollow handler — 204 or 404

php

private function unfollowUser(ServerRequestInterface $request): ResponseInterface
{
    $params     = (array) $request->getAttribute(Router::PARAMETERS_ATTRIBUTE);
    $followerId = isset($params['followerId']) && is_numeric($params['followerId'])
        ? (int) $params['followerId'] : 0;
    $followeeId = isset($params['followeeId']) && is_numeric($params['followeeId'])
        ? (int) $params['followeeId'] : 0;

    if ($followerId <= 0 || !$this->repo->findUserById($followerId)) {
        return $this->responseFactory->create(['error' => 'follower not found'], 404);
    }

    $removed = $this->repo->unfollow($followerId, $followeeId);

    if (!$removed) {
        return $this->responseFactory->create(['error' => 'not following'], 404);
    }

    return $this->responseFactory->createEmpty(204);
}

Note: createEmpty(204) — JsonResponseFactory has a dedicated method for body-less responses.

Stats endpoint

php

private function stats(ServerRequestInterface $request): ResponseInterface
{
    $params = (array) $request->getAttribute(Router::PARAMETERS_ATTRIBUTE);
    $userId = isset($params['userId']) && is_numeric($params['userId'])
        ? (int) $params['userId'] : 0;

    if ($userId <= 0 || !$this->repo->findUserById($userId)) {
        return $this->responseFactory->create(['error' => 'user not found'], 404);
    }

    return $this->responseFactory->create([
        'user_id'         => $userId,
        'followers_count' => $this->repo->followerCount($userId),
        'following_count' => $this->repo->followingCount($userId),
    ]);
}

AppFactory

php

final class AppFactory
{
    public static function createSqliteApp(string $dbPath): RequestHandlerInterface
    {
        $dbConfig = new DatabaseConfig(
            url: null,
            environment: 'test',
            adapter: 'sqlite',
            host: '',
            port: 1,
            name: $dbPath,
            user: '',
            password: '',
            charset: '',
        );

        $factory  = new PdoConnectionFactory($dbConfig);
        $executor = new PdoDatabaseQueryExecutor($factory);
        $psr17    = new Psr17Factory();
        $json     = new JsonResponseFactory($psr17, $psr17);

        $repo      = new FollowRepository($executor);
        $registrar = new RouteRegistrar($repo, $json);

        return new RuntimeApplicationFactory(
            $psr17,
            $psr17,
            routeRegistrars: [static fn(Router $r) => $registrar->register($r)],
        )->create();
    }
}

Testing strategy

Each test runs against a fresh in-memory SQLite file, created in setUp() and deleted in tearDown().

Key cases:

php

// Idempotent follow → 200
public function testFollowIdempotentReturns200(): void
{
    $alice = $this->createUser('Alice');
    $bob   = $this->createUser('Bob');

    $this->request('POST', "/users/{$alice}/follow", ['followee_id' => $bob]);
    $res = $this->request('POST', "/users/{$alice}/follow", ['followee_id' => $bob]);

    $this->assertSame(200, $res->getStatusCode());
    $this->assertTrue($this->json($res)['following']);
}

// Self-follow → 422
public function testFollowSelfReturns422(): void
{
    $alice = $this->createUser('Alice');
    $res   = $this->request('POST', "/users/{$alice}/follow", ['followee_id' => $alice]);
    $this->assertSame(422, $res->getStatusCode());
}

// Unfollow then re-follow → 201 again
public function testUnfollowThenRefollow(): void
{
    $alice = $this->createUser('Alice');
    $bob   = $this->createUser('Bob');

    $this->request('POST', "/users/{$alice}/follow", ['followee_id' => $bob]);
    $this->request('DELETE', "/users/{$alice}/follow/{$bob}");
    $res = $this->request('POST', "/users/{$alice}/follow", ['followee_id' => $bob]);

    $this->assertSame(201, $res->getStatusCode());
}

// Mutual follow
public function testMutualFollow(): void
{
    $alice = $this->createUser('Alice');
    $bob   = $this->createUser('Bob');

    $this->request('POST', "/users/{$alice}/follow", ['followee_id' => $bob]);
    $this->request('POST', "/users/{$bob}/follow", ['followee_id' => $alice]);

    $this->assertTrue($this->json($this->request('GET', "/users/{$alice}/is-following/{$bob}"))['following']);
    $this->assertTrue($this->json($this->request('GET', "/users/{$bob}/is-following/{$alice}"))['following']);
}

Order-by convention

Both listFollowers and listFollowing use ORDER BY f.id DESC — the most recently followed user appears first. This mirrors Twitter/Instagram's "followers" tab behaviour.

Common pitfalls

Pitfall	Fix
Returning 422 for unknown user in self-follow check	Validate user existence before the self-follow check
`followee_id` read from path instead of body	`followee_id` is in the request body; `followeeId` is the path param for DELETE
Using `createJson()`	The method is `create(array $data, int $status = 200)`
Using `createEmpty()` for a 200 with body	Only use `createEmpty()` for 204 No Content
Missing `JsonRequestBodyParser::parse()` call	Must call this manually in every handler that reads a JSON body

How to Build a User Follow System with NENE2 ​

What we're building ​

Database schema ​

API endpoints ​

Repository ​

Follow handler — idempotent POST ​

Unfollow handler — 204 or 404 ​

Stats endpoint ​

AppFactory ​

Testing strategy ​

Order-by convention ​

Common pitfalls ​