Question

I have a user table and a girls table and a girls_result table.

Basically I want only one user in USER entity, that USER is the device owner, and I want to call that same USER every time I need it.

USER can have multiple girls, and girls can have multiple results.
Data Model:
USER <------>>GIRLS<-------->>GIRLS_RESULTS

So I added user_deviceid attribute to USER Entity and created a singleton class so I can set a unique number for the user , and call it when I need it. I want to make sure multiple user objects with same UUID is not created.

@implementation SingletonClass

@synthesize singletonManagedObjectContext=_singletonManagedObjectContext;
@synthesize userIS=_userIS;

+ (SingletonClass *)sharedInstance
{
    static SingletonClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[SingletonClass alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

- (id)init {
    if (self = [super init]) {

        // set a singleton managed object context
        _singletonManagedObjectContext=self.singletonManagedObjectContext;

        //set only one user based on unique device id, so all tables will belong to that user
        NSManagedObjectContext *context = _singletonManagedObjectContext;

        //how to make sure that _userIS is the same user all the time???

        if (_userIS.user_deviceid == [self newUUID]) {
            NSLog(@"user dvice id matches");
        }
        else{
            _userIS.user_deviceid=[self newUUID];
        }


    }
    return self;
}

- (NSString *)newUUID
{
    CFUUIDRef theUUID = CFUUIDCreate(NULL);
    CFStringRef string = CFUUIDCreateString(NULL, theUUID);
    CFRelease(theUUID);
    return (NSString *)CFBridgingRelease(string);
} 

So How can I make sure that only one USER is created in USER entity and I can use that specific USER each time?

============EDIT ANSWER============
Based On Accepted Answer Below I have created a class it works fine

#import "CheckUser.h"
#import "USER.h"
#import "SingletonClass.h"

@implementation CheckUser


- (USER *)checkandreturnUser
{
    SingletonClass *sharedInstance = [SingletonClass sharedInstance];
    NSManagedObjectContext *context = sharedInstance.singletonManagedObjectContext;
    // Fetch Form Object
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:[NSEntityDescription entityForName:@"USER"
                                        inManagedObjectContext:context]];
    USER *user;
    NSError *error = nil;
    NSArray *userArray = [context executeFetchRequest:fetchRequest error:&error];
    NSLog(@"user array count %i",[userArray count]);
    if (userArray == nil) {
        NSAssert(0, @"There was an error fetching user object");
        userArray = [NSArray array];
    }
    // If user doesn't exist create it
    if ([userArray count] > 0) {
        NSAssert([userArray count] == 1, @"Expected one user but there were more");
        user = [userArray objectAtIndex:0];
    } else {
        //Create user object
        user = [NSEntityDescription insertNewObjectForEntityForName:@"USER"
                                             inManagedObjectContext:context];
        user.user_deviceid=[self newUUID];
        // maybe save here?
        if (![context save:&error]) {
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }
    }


    return user;
}
- (NSString *)newUUID
{
    CFUUIDRef theUUID = CFUUIDCreate(NULL);
    CFStringRef string = CFUUIDCreateString(NULL, theUUID);
    CFRelease(theUUID);
    return (NSString *)CFBridgingRelease(string);
}

Answer 1

The easiest solution is to probably check and make sure that you don’t create more than one USER. Once you have one user entity just stop.

[self newUUID] will always return a different value so _userIS.user_deviceid == [self newUUID] would always be NO

I would use an accessor for _userIS

- (NSManagedObject *)user
{
  if (_userIS) {
    return _userIS;
  }

  NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
  [fetchRequest setEntity:[NSEntityDescription entityForName:@"user" 
                                      inManagedObjectContext:_singletonManagedObjectContext]];
  NSError *error = nil;
  NSArray *userArray = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
  if (userArray == nil) {
    NSAssert(0, @"There was an error fetching user object");
    userArray = [NSArray array];
  }
  // If user doesn't exist create it
  if ([userArray count] > 0) { 
    NSAssert([userArray count] == 1, @"Expected one user but there were more");
    _userIS = [userArray objectAtIndex:0];
  } else {
    //Create user object
    _userIS = [NSEntityDescription insertNewObjectForEntityForName:@"user" 
                                                 inManagedObjectContext:_singletonManagedObjectContext];
    _userIS.user_deviceid=[self newUUID];
    // maybe save here?
  }
  [_userIS retain]; // Non-ARC only
  return _userIS;
}

Its also worth mentioning that having a root object is not uncommon and is part or the Model-View-Controller-Store pattern.