Sokil\Mongo\Document::isStored PHP Method

    public function isStored()
    {
        return $this->get('_id') && !$this->isModified('_id');
    }

 public function addRelation($relationName, Document $document)
 {
     if (!$this->isRelationExists($relationName)) {
         throw new \Exception('Relation "' . $relationName . '" not configured');
     }
     $relation = $this->relations[$relationName];
     list($relationType, $relatedCollectionName, $field) = $relation;
     $relatedCollection = $this->document->getCollection()->getDatabase()->getCollection($relatedCollectionName);
     if (!$relatedCollection->hasDocument($document)) {
         throw new \Sokil\Mongo\Exception('Document must belongs to related collection');
     }
     switch ($relationType) {
         case Document::RELATION_BELONGS:
             if (!$document->isStored()) {
                 throw new \Sokil\Mongo\Exception('Document ' . get_class($document) . ' must be saved before adding relation');
             }
             $this->document->set($field, $document->getId());
             break;
         case Document::RELATION_HAS_ONE:
             if (!$this->document->isStored()) {
                 throw new \Sokil\Mongo\Exception('Document ' . get_class($this) . ' must be saved before adding relation');
             }
             $document->set($field, $this->document->getId())->save();
             break;
         case Document::RELATION_HAS_MANY:
             if (!$this->document->isStored()) {
                 throw new \Sokil\Mongo\Exception('Document ' . get_class($this) . ' must be saved before adding relation');
             }
             $document->set($field, $this->document->getId())->save();
             break;
         case Document::RELATION_MANY_MANY:
             $isRelationListStoredInternally = isset($relation[3]) && $relation[3];
             if ($isRelationListStoredInternally) {
                 $this->document->push($field, $document->getId())->save();
             } else {
                 $document->push($field, $this->document->getId())->save();
             }
             break;
         default:
             throw new \Sokil\Mongo\Exception('Unsupported relation type "' . $relationType . '" when resolve relation "' . $relationName . '"');
     }
     return $this;
 }