private function sync($id, $attributes)
    {
        // Return if the ID is invalid, or attributes is not an array
        if (!isset($id) || !isset($attributes) || empty($attributes)) {
            return $this;
        }

        // Return if there are no relationships of that type
        if (!isset($this->relationships['belongsToMany']) || empty($this->relationships['belongsToMany'])) {
            return $this;
        }

        // Sync each relationship
        foreach ($this->relationships['belongsToMany'] as $join) {

            // The sync option has to be true, the related attribute an array
            if (isset($join['sync']) && $join['sync'] && isset($attributes[$join['alias']]) && is_array($attributes[$join['alias']])) {

                // Delete old values
                $delete = $this->db()->table($join['pivot'])->where($join['localKey'], $id, '=')->delete();

                // Insert new values
                foreach ($attributes[$join['alias']] as $value) {
                    $this->db()->table($join['pivot'])->insert([$join['localKey'] => $id, $join['foreignKey'] => $value]);
                }
            }
        }

        // Return model instance
        return $this;
    }