bug with phpunit

FirstModel.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class FirstModel extends Model
{
    protected $table = 'FirstModels';

    public $timestamps = false;

    protected $connection = 'db1';

}

SecondModel.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class SecondModel extends Model
{
    public function firstModels()
    {
        $defaultDb       = config('database.default');
        $db              = config('database.connections.' . $defaultDb . '.database');
        return $relation = $this->belongsToMany(
            \App\FirstModel::class,
            $db . '.secound_model_First_model',
            'second_model_id',
            'first_model_id'
        )->withPivot('type');
    }

}