BMO

/* MyBridge.swift */

private lazy var restMapper: RESTMapper<NSEntityDescription, NSPropertyDescription> = {
   let userMapping: [_RESTConvenienceMappingForEntity] = [
      .restPath("/users(/|username|)"),
      .uniquingPropertyName("bmoId"),
      .propertiesMapping([
         "bmoId":     [.restName("id")],
         "username":  [.restName("user_name")],
         "firstname": [.restName("first_name")],
         "age":       [.restName("age"),       .restToLocalTransformer(RESTIntTransformer())]
      ])
   ]
       
   return RESTMapper(
      model: dbModel,
      defaultPaginator: RESTOffsetLimitPaginator(),
      convenienceMapping: [
         "User": userMapping
      ]
   )
}()

/* MyBridge.swift */

func remoteObjectRepresentations(fromFinishedOperation operation: BackOperationType, userInfo: UserInfoType) throws -> [RemoteObjectRepresentationType]? {
   /* In our case, the operation has a results property containing either the
    * parsed JSON from the API or an error. */
   switch operation.results {
   /* We access the "items" elements because our API returns the objects in this key. 
    * The behaviour may be different with another API. */
   case .success(let success): return success["items"] as? [MyBridge.RemoteObjectRepresentationType]
   case .error(let e):         throw Err.operationError(e)
   }
}

/* MyBridge.swift */

func mixedRepresentation(fromRemoteObjectRepresentation remoteRepresentation: RemoteObjectRepresentationType, expectedEntity: DbType.EntityDescriptionType, userInfo: UserInfoType) -> MixedRepresentation<DbType.EntityDescriptionType, RemoteRelationshipAndMetadataRepresentationType, UserInfoType>? {
   /* First let’s get which entity the remote representation represents.
    * The REST mapper will do this job for us. */
   guard let entity = restMapper.actualLocalEntity(forRESTRepresentation: remoteRepresentation, expectedEntity: expectedEntity) else {return nil}

   /* The REST mapper does not know about the MixedRepresentation
    * structure, but can convert a remote representation into a Dictionary
    * that we will use to build the MixedRepresentation instance we want. */
   let mixedRepresentationDictionary = restMapper.mixedRepresentation(ofEntity: entity, fromRESTRepresentation: remoteRepresentation, userInfo: userInfo)

   /* We need to use the REST mapper once again to retrieve the uniquing
    * id from the Dictionary we created above. */
   let uniquingId = restMapper.uniquingId(forLocalRepresentation: mixedRepresentationDictionary, ofEntity: entity)
   /* Finally, with everything we have retrieved above, we can create the
    * MixedRepresentation instance that we return to the caller. */
   return MixedRepresentation(entity: entity, uniquingId: uniquingId, mixedRepresentationDictionary: mixedRepresentationDictionary, userInfo: userInfo)
}

/* AppDelegate.swift */

import BMO

class AppDelegate : NSObject, UIApplicationDelegate {

   private(set) var requestManager: RequestManager!

   func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
      /* Setup BMO request manager */
      requestManager = RequestManager(bridges: [YourBridge(dbModel: container.managedObjectModel)], resultsImporterFactory: BMOBackResultsImporterForCoreDataWithFastImportRepresentationFactory())
   }

   /* A struct to help BMO. We are actually working on several solutions
    * to avoid the use of this one. */
   private struct BMOBackResultsImporterForCoreDataWithFastImportRepresentationFactory : AnyBackResultsImporterFactory {

      func createResultsImporter<BridgeType : Bridge>() -> AnyBackResultsImporter<BridgeType>? {
         return (AnyBackResultsImporter(importer: BackResultsImporterForCoreDataWithFastImportRepresentation<YourBridge>(uniquingPropertyName: "bmoId")) as! AnyBackResultsImporter<BridgeType>)
      }

   }

}

/* ViewController.swift */

private func refreshUser(username: String) {
   let fetchRequest: NSFetchRequest<User> = User.fetchRequest()
   fetchRequest.predicate = NSPredicate(format: "%K != %@", #keyPath(User.username), username)

   let context = AppDelegate.shared.context!
   _ = AppDelegate.shared.requestManager!.fetchObject(
      fromFetchRequest: fetchRequest as! NSFetchRequest<NSFetchRequestResult>,
      fetchType: .always, onContext: context, handler: { (user: User?, fullResponse: AsyncOperationResult<BridgeBackRequestResult<YourBridge>>) -> Void in
         /* Use the fetched user here. */
      }
   )
}