/**
  * append a model to the list and correctly set any fields required to make this re-fetchable through the same list.
  *
  * @param \Rhubarb\Stem\Models\Model $model
  * @return \Rhubarb\Stem\Models\Model|null
  */
 public function append(Model $model)
 {
     $result = null;
     // If the list was filtered make sure that value is set on the model.
     if ($this->filter !== null) {
         $result = $this->filter->setFilterValuesOnModel($model);
     }
     $model->save();
     // Make sure the list has been fetched so we can pop the unique identifier on the end.
     if ($this->fetched) {
         $this->uniqueIdentifiers[] = $model->UniqueIdentifier;
     }
     return $result === null ? $model : $result;
 }