/**
  * Transfer all back refs from given object of same class
  *
  * @param CMbObject &$object The object to transfer back objects from
  *
  * @return string|null store-like error message if failed, null if successful
  */
 function transferBackRefsFrom(CMbObject &$object)
 {
     if (!$object->_id) {
         trigger_error("transferNoId");
     }
     if ($object->_class !== $this->_class) {
         trigger_error("An object from type '{$object->_class}' can't be merge with an object from type '{$this->_class}'", E_USER_ERROR);
     }
     $object->loadAllBackRefs();
     foreach ($object->_back as $backName => $backObjects) {
         /** @var self[] $backObjects */
         if (count($backObjects)) {
             $backSpec = $this->_backSpecs[$backName];
             $backObject = new $backSpec->class();
             $backField = $backSpec->field;
             $fwdSpec = $backObject->_specs[$backField];
             $backMeta = $fwdSpec->meta;
             // Change back field and store back objects
             foreach ($backObjects as $backObject) {
                 /** @var self $transferer Dummy tranferer object to prevent checks on all values */
                 $transferer = new $backObject->_class();
                 $transferer->_id = $backObject->_id;
                 $transferer->{$backField} = $this->_id;
                 $transferer->_forwardRefMerging = true;
                 // Cas des meta objects
                 if ($backMeta) {
                     $transferer->{$backMeta} = $this->_class;
                 }
                 if ($msg = $transferer->store()) {
                     return $msg;
                 }
             }
         }
     }
     return null;
 }