class Chat extends Model
{
    protected $primaryKey = 'chat_id';
    protected $guarded    = [];

    public function comment()
    {
        return $this->hasOne(Comments::class, 'comment_id', 'comment_id');
    }
}

class Comments extends Model
{
    protected $primaryKey = 'comment_id';
    protected $guarded    = [];
}

public function up()
{
    Schema::create('comments', function (Blueprint $table) {
        $table->increments('comment_id');
        $table->integer('user_id')->unsigned()->index('comments_user_id');
        $table->boolean('mark');
        $table->string('nick_name', 15)->nullable();
        $table->text('comment', 65535);
    });
}

$comment = Comments::create([
                'user_id' => 1,
                'mark' => 50,
                'comment' => 'Lorem ipsum dolor es at'
            ]); // $comment->comment_id = 1

$comment = $chat->comment()->create([
                'user_id'    => 1,
                'mark'       => 50,
                'comment'    => 'Lorem ipsum dolor es at',
            ]); // $comment->comment_id = 0