Sokil\Mongo\Document::addToSet PHP Method

    public function addToSet($fieldName, $value)
    {
        $oldValues = $this->get($fieldName);
        if (!$oldValues) {
            $newValue = array($value);
            $this->operator->addToSet($fieldName, $value);
        } elseif (!is_array($value)) {
            if ($oldValues === $value) {
                return $this;
            }
            $newValue = array($oldValues, $value);
            $this->operator->set($fieldName, $newValue);
        } else {
            if (in_array($value, $oldValues)) {
                return $this;
            }
            $newValue = array_merge($oldValues, array($value));
            $this->operator->addToSet($fieldName, $value);
        }
        parent::set($fieldName, $newValue);
        return $this;
    }