Fetched managedObject return null - objective-c

Question

I created an IBAction that I link to saveButton, when I call all core data methods inside the IBAction it works fine in following code:
- (IBAction)saveButtonTap:(id)sender{
NSManagedObjectContext *context = [self managedObjectContext];
NSManagedObject *object = [NSEntityDescription insertNewObjectFor:#"Person" inManagedContext:context];
[self setValue:firstNameTextField.text forKey:#"firstName"];
[self setValue:lastNameTextField.text forKey:#"lastName"];
NSError *error = nil;
if (![context save:&error])
NSLog(#"Can't save transaction - %# || %# ", error, [error localizedDescription]);
}
And when I separate the core data saving methods as the below, the saved firstName and lastName, showing null when fetched. It didn't return error, but I have no idea after googling all day. Could someone point anything wrong with my code?
Person.h + Person.m
#interface Person : NSManagedObject
#property(strong) NSString *firstName;
#property(strong) NSString *lastName;
- (void)save;
#end
#implementation Person
#synthesize firstName;
#synthesize lastName;
- (void)save{
[self setValue:self.firstName forKey:#"firstName"];
[self setValue:self.lastName forKey:#"lastName"];
}
#end
mainViewController.h + mainViewController.m
#interface mainViewController : UIViewController
#property(strong) ManagedObjectContext *context;
#property(strong) Person *person;
- (IBAction)saveButtonTap:(id)sender;
#end
#implementation mainViewController
#synthesize context;
#synthesize person;
- (void)viewDidLoad{
// ... some view did Load rituals
if(context == nil){
context = [self managedObjectContext]; // Assume this method calls for managed object context from shared application.
}
if(person == nil){
person = [NSEntityDescription insertNewObjectFor:#"Person" inManagedContext:context];
}
}
- (IBAction)saveButtonTap:(id)sender{
person.firstName = firstNameTextField.text;
person.lastName = lastNameTextField.text;
[self.person save];
NSError *error = nil;
if (![context save:&error])
NSLog(#"Can't save transaction - %# || %# ", error, [error localizedDescription]);
}
#end

Answer 1

There is no need of save function for Person.
person.firstName = firstNameTextField.text
is equivalent to
[self setValue:self.firstName forKey:#"firstName"];
What you need, is just this:
(IBAction)saveButtonTap:(id)sender{
person.firstName = firstNameTextField.text;
person.lastName = lastNameTextField.text;
NSError *error = nil;
if (![context save:&error])
NSLog(#"Can't save transaction - %# || %# ", error, [error localizedDescription]);
}
But, I am not sure whether this is the cause of the issue. Try it and let me know.