ExampleMapping.swift

let pred = NSPredicate(format: "slug = 'default'")
if let item: FolderData = databaseProvider.firstObject(type: DataModelType.folderData, predicate: pred) {
	let realm = RealmConfigurator.shared.reamInstance()
      
	try! realm.write {
		item.mapping(map: Map(mappingType: .fromJSON, JSON: ["test": "Whaaaat?"]))
	}
}

FolderData.swift

class FolderData: ObjectMappable, PonsoConvertibleProtocol {
  dynamic var slug: String?
  dynamic var title = ""
  dynamic var test = ""
  var statuses = List<StatusData>()
  
  required convenience init?(map: Map) {
    self.init()
  }
  
  // Mappable
  override func mapping(map: Map) {
    super.mapping(map: map)
    
    if map.mappingType == .fromJSON {
      if slug == nil {
        slug <- map["slug"]
      }
    } else {
      slug >>> map["slug"]
    }
    
    title <- map["title"]
    statuses <- (map["statuses"], ListTransform<StatusData>())
    test <- map["test"]
  }
  
  override class func primaryKey() -> String {
    return "slug"
  }
}

Обновится только значение Test. Значение Title останется прежним