- ['required', Rule::exists('circles')] // ❌ circle_id: integer (incorrectly maps to column circle_id)
- ['required', Rule::exists('circles', 'id')] // ❌ circle_id: integer
- ['required', 'exists:circles'] // ❌ circle_id: integer
- ['required', 'exists:circles,id'] // ❌ circle_id: integer

Schema::create('circles', function (Blueprint $table) {
    $table->uuid('id')->primary();
    $table->foreignUuid('creator_id')
        ->constrained('users')
        ->restrictOnDelete();
    $table->string('name');
    $table->text('description')->nullable();
    $table->string('emoji')->nullable();
    $table->timestamps();
});

<?php

declare(strict_types=1);

namespace App\Models;

use Illuminate\Database\Eloquent\Concerns\HasUuids;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;
use Illuminate\Database\Eloquent\Relations\BelongsToMany;
use Illuminate\Database\Eloquent\Relations\HasMany;
use Spatie\MediaLibrary\HasMedia;
use Spatie\MediaLibrary\InteractsWithMedia;

/**
 * Circle aggregates members, media, and invitations.
 */
final class Circle extends Model implements HasMedia
{
    use HasFactory;
    use HasUuids;
    use InteractsWithMedia;

    public const MEDIA_COLLECTION_AVATAR = 'avatar';

    protected function casts(): array
    {
        return [
            'created_at' => 'immutable_datetime',
            'updated_at' => 'immutable_datetime',
        ];
    }

    public function creator(): BelongsTo
    {
        return $this->belongsTo(User::class, 'creator_id');
    }

    public function memberships(): HasMany
    {
        return $this->hasMany(CircleMembership::class);
    }

    public function members(): BelongsToMany
    {
        return $this->belongsToMany(User::class, 'circle_user')
            ->using(CircleMembership::class)
            ->withPivot('id', 'role', 'created_at', 'updated_at')
            ->withTimestamps();
    }

    public function invitations(): HasMany
    {
        return $this->hasMany(CircleInvitation::class);
    }

    public function registerMediaCollections(): void
    {
        $this->addMediaCollection(self::MEDIA_COLLECTION_AVATAR)
            ->singleFile();
    }
}