public function addField(PhabricatorCustomField $field)
 {
     $role_storage = PhabricatorCustomField::ROLE_STORAGE;
     if (!$field->shouldEnableForRole($role_storage)) {
         return $this;
     }
     $storage = $field->newStorageObject();
     $source_key = $storage->getStorageSourceKey();
     $this->fieldMap[$source_key][] = $field;
     if (empty($this->storageSources[$source_key])) {
         $this->storageSources[$source_key] = $storage;
     }
     return $this;
 }