iOS-Swift : Serialize a NSManagedObject to JSON (with relationships)

Question

After spending too much time trying to find the best practices, here I am once again asking for some help, hoping I am not the only one struggling with this :

I have NSManaged Objects like these :

import Foundation
import CoreData

class Credential: NSManagedObject {

    @NSManaged var credentialArrivalDate: String?
    @NSManaged var credentialBarCode: String?
    @NSManaged var credentialComment: String?
    @NSManaged var credentialCountCheckIn: Int
    @NSManaged var credentialCountCheckOut: Int
    @NSManaged var credentialDepartureDate: String?
    @NSManaged var credentialId: String?
    @NSManaged var credentialIsArrived: Bool
    @NSManaged var credentialIsCheckedOut: Bool
    @NSManaged var credentialIsHost: Bool
    @NSManaged var credentialLastModificationDate: String?
    @NSManaged var credentialMemberControlled: String?
    @NSManaged var credentialNumberOfPeople: Int
    @NSManaged var credentialRSVP: Int
    @NSManaged var credentialTypeId: String?
    @NSManaged var credentialTypeName: String?
    @NSManaged var credentialZoneId: String?
    @NSManaged var credentialZoneName: String?
    @NSManaged var credentialHosts: Set<Host>?
    @NSManaged var credentialMember: Member
    @NSManaged var credentialExtensionFields: Set<ExtensionFields>?
    @NSManaged var credentialDeltaFirstCheckIn: String?
    @NSManaged var credentialDeltaFirstCheckOut: String?
    @NSManaged var credentialIsCheckedIn: Bool

}

I have a relationship with a Member Entity :

import Foundation
import CoreData

class Member: NSManagedObject {

    @NSManaged var memberCompany: String
    @NSManaged var memberEMail: String
    @NSManaged var memberFirstName: String
    @NSManaged var memberId: String
    @NSManaged var memberJob: String
    @NSManaged var memberLastModificationDate: String
    @NSManaged var memberLastName: String
    @NSManaged var memberPhoneNumber: String
    @NSManaged var memberTitle: String
    @NSManaged var memberCredential: Credential

}

What I am trying to do is to serialize my Credential object to JSON using NSJSONSerialization.dataWithJSONObject to generate my JSON and POST it to a Server.

However, I've got an error while serializing my object, and I believe this is because my object does not respect this (from Apple documentation) :

An object that may be converted to JSON must have the following properties:

• The top level object is an NSArray or NSDictionary.
• All objects are instances of NSString, NSNumber, NSArray, NSDictionary, or NSNull.
• All dictionary keys are instances of NSString.
• Numbers are not NaN or infinity.

So how can I Serialize my Object to JSON ? Should I find some way to convert a Member to a Dictionary, then try to find a way to set this dictionary as the value for my relationship with a credential ?

Have I missed a built-in mapper that handles it ?

I am sorry if that is not clear but any help would be very nice.

In advance, thank you.

Hugo

EDIT :

As suggested in an answer, I tried to fetch a dictionary instead of a NSManagedObject but the logged dictionary is empty (however, the same code worked for a very simple class with a few string attributes and no relationship), here is the code :

/*******************************
    TEST : Fetch dictionary instead of NSManagedObject

    */

    // create Credential entity
    let newCredential = NSEntityDescription.insertNewObjectForEntityForName("Credential", inManagedObjectContext: self.managedObjectContext!) as! Credential

    // create Credential entity
    let newMember = NSEntityDescription.insertNewObjectForEntityForName("Member", inManagedObjectContext: self.managedObjectContext!) as! Member

    // instanciate some fields
    newCredential.credentialId = "44444"
    newMember.memberFirstName = "TEST"

    // set the relationship
    newCredential.credentialMember = newMember

    // retrieve currentCredential
    let requestGuest = NSFetchRequest(entityName: "Credential")

    // get a dictionary instead of NSManagedObject
    requestGuest.resultType = NSFetchRequestResultType.DictionaryResultType

    let fetchedGuest = (managedObjectContext!.executeFetchRequest(requestGuest, error:nil))! // no need to cast here ?

    println(fetchedGuest.description)

    /*******************************/

Answer 1

I tried the same but could not get succeed with one to one and one to many relationships. So I wrote a parser to convert managed objects to dictionary and array. You may use it;

//Tested on xCode 6.4 - Swift 1.2
let requestGuest = NSFetchRequest(entityName: "Credential")
let fetchedGuest = (managedObjectContext!.executeFetchRequest(requestGuest, error:nil))!

let dataInArr:NSArray = ManagedParser.convertToArray(fetchedGuest);
NSLog("dataInArr \(dataInArr)");

//--

//
//  ManagedParser.swift
//  Created by Shoaib on 7/29/15.
//

import UIKit
import CoreData

class ManagedParser: NSObject {


    class func convertToArray(managedObjects:NSArray?, parentNode:String? = nil) -> NSArray {

        var rtnArr:NSMutableArray = NSMutableArray();

        if let managedObjs:[NSManagedObject] = managedObjects as? [NSManagedObject] {
            for managedObject:NSManagedObject in managedObjs {
                rtnArr.addObject(self.convertToDictionary(managedObject, parentNode: parentNode));
            }
        }

        return rtnArr;
    } //F.E.

    class func convertToDictionary(managedObject:NSManagedObject, parentNode:String? = nil) -> NSDictionary {

        var rtnDict:NSMutableDictionary = NSMutableDictionary();

        let entity:NSEntityDescription = managedObject.entity;
        let properties:[String] = (entity.propertiesByName as NSDictionary).allKeys as! [String];

        let parentNode:String = parentNode ?? managedObject.entity.name!;
        for property:String in properties  {
            if (property.caseInsensitiveCompare(parentNode) != NSComparisonResult.OrderedSame)
            {
                let value: AnyObject? = managedObject.valueForKey(property);

                if let set:NSSet = value as? NSSet  {
                    rtnDict[property] = self.convertToArray(set.allObjects, parentNode: parentNode);
                } else if let vManagedObject:NSManagedObject = value as? NSManagedObject {
                    if (vManagedObject.entity.name != parentNode) {
                        rtnDict[property] = self.convertToDictionary(vManagedObject, parentNode: parentNode);
                    }
                } else  if let vData:AnyObject = value {
                    rtnDict[property] = vData;
                }
            }
        }

        return rtnDict;
    } //F.E.


} //CLS END

Answer 2

You could achieve this using a third-party framework like Groot or Sync.