public function getKey()
    {
        $key = false;

        if (count($this->where) == 1) {
            $where = array_values($this->where);
            $where = array_pop($where);

            if (isset($where['property'])) {
                $property = $where['property'];
                $keys = $this->getRepository()->getDescription()->getIdentifyingProperties();
                if (isset($keys[$property]) && isset($where['constraint']) && $where['constraint'] == '=' && !is_array($where['value'])) {
                    $key[$property] = $where['value'];
                }
            }
        }

        return $key;
    }