Problem updating document set of existing doc

gistfile1.aw

<? 
/**
* Represents the invite system. The invite collection contains one document for each invited email address.
**/ 
class Model_Mongo_Invite extends Shanty_Mongo_Document {
    
    protected static $_db = 'contactzilla';
    protected static $_collection = 'invite';
    
    protected static $_requirements = array(
        'secret' => 'Required',
        'email' => 'Required',
        'companies' => array('DocumentSet'),
        'companies.$' => array('Document:Model_Mongo_CompanyInvite')
    );
	
	/**
    * We've overridden save to catch urlKey clash.
    **/
    public function save() {
        
        try {
            parent::save();
        } catch(Exception $e) {
            
            $m = $e->getMessage();
            
            //We might have had a email clash, in which case we just lookup the existing doc and save.
            if (strpos($m,'duplicate') && strpos($m,'email')) { 
                
                //Get companies that have been added.
                $companies = $this->companies;

                //Save it to existing document.
                $i = Model_Mongo_Invite::one(array('email' => $this->email));

                foreach($this->companies as $ca) {
                    $uca = $i->companies->new();
                    $uca->company = $ca->company;
                    $uca->addedByUser = $ca->user;
                }

                $i->save();
                
            } else {
                throw $e;
            }

        }
        
    }
    
}

class Model_Mongo_CompanyInvite extends Shanty_Mongo_Document {
    
    protected static $_requirements = array(
        'company' => array('Document:Model_Mongo_Company', 'AsReference'),
        'addedByUser' => array('Document:Model_Mongo_User', 'AsReference')
    );

	protected function preSave() {
	    
	    if($this->isNewDocument()) {
            $this->createdAt = time();
	    }
	    
	}
	
}