Sokil\Mongo\Document::unsetField PHP Method

    public function unsetField($fieldName)
    {
        if (!$this->has($fieldName)) {
            return $this;
        }
        parent::unsetField($fieldName);
        if ($this->getId()) {
            $this->operator->unsetField($fieldName);
        }
        return $this;
    }

 public function removeRelation($relationName, Document $document = null)
 {
     if (!$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 ($document && !$relatedCollection->hasDocument($document)) {
         throw new \Sokil\Mongo\Exception('Document must belongs to related collection');
     }
     switch ($relationType) {
         case Document::RELATION_BELONGS:
             $this->document->unsetField($field)->save();
             break;
         case Document::RELATION_HAS_ONE:
             $document = $this->getRelated($relationName);
             if (!$document) {
                 // relation not exists
                 return $this;
             }
             $document->unsetField($field)->save();
             break;
         case Document::RELATION_HAS_MANY:
             if (!$document) {
                 throw new \Sokil\Mongo\Exception('Related document must be defined');
             }
             $document->unsetField($field)->save();
             break;
         case Document::RELATION_MANY_MANY:
             if (!$document) {
                 throw new \Sokil\Mongo\Exception('Related document must be defined');
             }
             $isRelationListStoredInternally = isset($relation[3]) && $relation[3];
             if ($isRelationListStoredInternally) {
                 $this->document->pull($field, $document->getId())->save();
             } else {
                 $document->pull($field, $this->document->getId())->save();
             }
             break;
         default:
             throw new \Sokil\Mongo\Exception('Unsupported relation type "' . $relationType . '" when resolve relation "' . $relationName . '"');
     }
     return $this;
 }