Paradox/pod/Edge.php
public function getToKey()
    {
        if (isset($this->_to)) {
            return $this->_to->getPod()->getKey();
        } elseif (isset($this->_data['_to'])) {
            return $this->parseIdForKey($this->_data['_to']);
        } else {
            return null;
        }
    }
public function getToKey()
    {
        if (isset($this->_to)) {
            return $this->_to->getPod()->getKey();
        } elseif (isset($this->_data['_to'])) {
            return $this->parseIdForKey($this->_data['_to']);
        } else {
            return null;
        }
    }
Get the key (id without the collection) of the to node by checking for it in the actual _to property or checking an internal property.
This is used because when documents are retrieved from the server, we will not have a model in the _to property.
This allows us to lazy load any vertices we need later.
@return null|string