Core Data Edit Attributes - iphone

Question

So im really new to core data, but i went through a tutorial and pretty much understand it, well at least the idea behind most of the things. But I still have 1 question that i cant find anywhere. It seems really simple but here it is. If I were to have two strings inside one entity lets say:
1.name
2.position
If the name is already entered how might i allow a user to enter text into a textField and assign it to their position at a later time? Even if there were 20 names, considering no duplicates?
I was thinking it might be something like this...But it doesnt seem to work.
UserInfo *userInfo = (UserNumber *)[NSEntityDescription insertNewObjectForEntityForName:#"UserInfo" inManagedObjectContext:managedObjectContext];
if ([userName isEqualToString:"#James"]) {
userInfo.Position = nameField.text;
}

Answer 1

On the code above you are casting (UserNumber*) to an object that you are declaring as (UserInfo*)? Which is what and is there any reason why you are doing that?
If I understand your question correctly, you want to create a record with only the username pre-populated and then allow that record to be updated at a later stage.
I will assume your entity is called UserInfo and that there are 2 NSString properties created for it - userName and position. I also assume you have created the class files for UserInfo and imported the header into the relevant view controllers.
Here's how you would do it:
1) Firstly, assuming you have username typed in a UITextField *userNameField, let's create a new record.
UserInfo *userInfo = (UserInfo*)[NSEntityDescription insertNewObjectForEntityForName:#"UserInfo" inManagedObjectContext:self.managedObjectContext];
[userInfo setValue:userNameField.text forKey:#"userName"];
This will create a new instance of UserInfo in your managed object context and set the value of userName to the value on userNameField.text
Then at a later stage a user will get to a point where they can update their records in your app (you may need to think about authentication somewhere here). You will fetch the record that matches your specified username:
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSPredicate *userNamePredicate = [NSPredicate predicateWithFormat:#"(userName == %#)", userNameField.text];
[fetchRequest setPredicate:userNamePredicate];
NSEntityDescription *userInfo = [NSEntityDescription entityForName:#"UserInfo" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:userInfo];
NSError *error;
NSArray *fetchRequestArray = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
[fetchRequest release];
If the fetchRequest found match(es) to your userNameField.text paramater, they will be saved in the fetchRequestArray. There should only be a maximum of one object there if you take the necessary steps to make the userName property mandatory AND unique.
Access the object by grabbing the objectAtIndex:0 in the array and change it's position property:
UserInfo *userInfoToBeEdited = [fetchRequestArray objectAtIndex:0];
[userInfoToBeEdit setValue:positionTextField.text forKey:#"position"];
In both cases above, remember to invoke CoreData's save method when you are ready to commit your changes. Before save is invoked your changes are only kept in your managed object context which is basically a scratch pad for your persistent data.
[EDIT TO ADD SAVE METHOD]
As per your comment, I usually have the save method below in my AppDelegate (copy/paste directly from Apple template)
- (void)saveContext
{
error = nil;
NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
if (managedObjectContext != nil)
{
if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error])
{
[self seriousErrorAlert];
}
}
}
And then whenever I need to save changes, from any view controller I simply grab a reference to my AppDelegate and fire it off:
AppDelegate *theDelegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];
[theDelegate saveContext];