Paradox/pod/Edge.php
public function getFromKey()
    {
        if (isset($this->_from)) {
            return $this->_from->getPod()->getKey();
        } elseif (isset($this->_data['_from'])) {
            return $this->parseIdForKey($this->_data['_from']);
        } else {
            return null;
        }
    }
public function getFromKey()
    {
        if (isset($this->_from)) {
            return $this->_from->getPod()->getKey();
        } elseif (isset($this->_data['_from'])) {
            return $this->parseIdForKey($this->_data['_from']);
        } else {
            return null;
        }
    }
Get the key (id without the collection) of the to node by checking for it in the actual _from 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 _from property.
This allows us to lazy load any vertices we need later.
@return null|string