How to Build an Emoji Reaction System with NENE2

This guide walks through building a reaction system where users react to posts with emojis, with grouped counts and per-user reaction tracking.

Field Trial: FT143
NENE2 version: ^1.5
Covered topics: UNIQUE(post_id, user_id, emoji) constraint, GROUP BY emoji counts, per-user reaction tracking, emoji length validation, MySQL integration tests

---

What we're building

• POST /posts — create a post
• POST /posts/{id}/reactions — add a reaction (emoji string, one per emoji per user)
• DELETE /posts/{id}/reactions/{emoji} — remove a reaction (own only)
• GET /posts/{id}/reactions — get reaction counts and current user's reactions

---

Database schema

CREATE TABLE reactions (
    id         INTEGER PRIMARY KEY AUTOINCREMENT,
    post_id    INTEGER NOT NULL,
    user_id    INTEGER NOT NULL,
    emoji      TEXT    NOT NULL,
    created_at TEXT    NOT NULL,
    UNIQUE (post_id, user_id, emoji),
    FOREIGN KEY (post_id) REFERENCES posts(id),
    FOREIGN KEY (user_id) REFERENCES users(id)
);

UNIQUE (post_id, user_id, emoji) — one row per emoji per user per post. The same user can react with different emojis (👍 and ❤️ = 2 rows). Multiple users can use the same emoji (each gets their own row).

---

Duplicate reaction → 409

public function addReaction(int $postId, int $userId, string $emoji, string $now): bool
{
    try {
        $this->executor->execute(
            'INSERT INTO reactions (post_id, user_id, emoji, created_at) VALUES (?, ?, ?, ?)',
            [$postId, $userId, $emoji, $now],
        );
        return true;
    } catch (DatabaseConstraintException) {
        return false;
    }
}

The handler returns 409 when addReaction() returns false. No separate existence check needed.

---

Grouped reaction counts with GROUP BY

SELECT emoji, COUNT(*) as cnt
FROM reactions
WHERE post_id = ?
GROUP BY emoji
ORDER BY cnt DESC, emoji ASC

Sorted by count descending (most popular emoji first), then alphabetically as a tiebreaker. The result maps directly to a PHP array<string, int>:

$counts = [];
foreach ($rows as $row) {
    $arr = (array) $row;
    if (isset($arr['emoji']) && is_string($arr['emoji'])) {
        $counts[$arr['emoji']] = isset($arr['cnt']) ? (int) $arr['cnt'] : 0;
    }
}

---

Per-user reactions (optional actor)

The GET /reactions endpoint accepts an optional X-User-Id header. When present, the response includes the list of emojis the caller has used:

$actorId       = (int) $request->getHeaderLine('X-User-Id');
$userReactions = $actorId > 0 ? $this->repository->getUserReactions($postId, $actorId) : [];

This allows the UI to show which emojis the current user has already reacted with.

---

Emoji validation

if (mb_strlen($emoji) > 8) {
    return $this->responseFactory->create(['error' => 'emoji too long'], 422);
}

mb_strlen counts Unicode code points, not bytes. A single emoji like 🧑‍💻 (person: technologist) is 3 code points; an 8-char limit accommodates most emoji sequences. Adjust to your requirements.

---

MySQL integration tests (FT143)

MySQL teardown order matters:

$this->pdo->exec('SET FOREIGN_KEY_CHECKS = 0');
$this->pdo->exec('DROP TABLE IF EXISTS reactions');
$this->pdo->exec('DROP TABLE IF EXISTS posts');
$this->pdo->exec('DROP TABLE IF EXISTS users');
$this->pdo->exec('SET FOREIGN_KEY_CHECKS = 1');

MySQL schema uses VARCHAR(32) for emoji (not TEXT) to allow the column in a UNIQUE key without a prefix length. VARCHAR(32) stores up to 32 characters, which covers all emoji sequences.

---

Common pitfalls

Pitfall	Fix
Allowing duplicate emoji reactions	UNIQUE (post_id, user_id, emoji) + catch DatabaseConstraintException
Using strlen() for emoji length	Use mb_strlen() — emoji are multi-byte Unicode
Mutable count column gets out of sync	Count from reactions table with GROUP BY emoji
Missing MySQL emoji support	Use utf8mb4 charset and VARCHAR (not CHAR) for emoji column
is_array() on fetchAll result is always true	Skip the check; fetchAll already returns array<int, array<string, mixed>>