Sokil\Mongo\Operator::addToSet PHP Method

    public function addToSet($field, $value)
    {
        // new field
        if (!isset($this->operators['$addToSet'][$field])) {
            $this->operators['$addToSet'][$field] = $value;
            return $this;
        }
        // scalar value or array in existed field
        if (!is_array($this->operators['$addToSet'][$field]) || !isset($this->operators['$addToSet'][$field]['$each'])) {
            $this->operators['$addToSet'][$field] = array('$each' => array($this->operators['$addToSet'][$field], $value));
            return $this;
        }
        // field already $each
        $this->operators['$addToSet'][$field]['$each'][] = $value;
        return $this;
    }