make a unique attribute

I'm having a problem to make unique attributes !

I am already searched here for methods that you guys makes an attribute unique in XCode programming:

1: using if(![ARRAY containsObject:object]) ...

2: fetch Array with object and then if ([object count]>0) ... 

My situation :

I have an Entity called userAccounts that is contains 2 attribute : userName & passWord .

I have an ArrayController that is bind to managedObjectContext and set to the Entity userAccounts

I have 2 textfield called userNameTF and passwordTF and all I want is to find if the userNameTF string value is added before to the UserAcco开发者_如何学运维unts or not ( uniqueness propose ).

the problem is :

1. NSArrayController does not support containsObject method

2. I can't load userName attributes to an Array to use it for containsObject method

what should I do to solve this problem?

Many thanks in Advance

---

You can get an array containing the value of the userName property in all of the objects in the array controller using arrangedObjects and valueForKey::

NSArray *userNames = [[arrayController arrangedObjects] valueForKey:@"userName"];
if([userNames containsObject:@"theNewUsername"]) {
    // user name taken
} else {
    // user name available
}

You could also use indexOfObjectPassingTest: if you want to find the object which already has the username:

NSArray *objects = [arrayController arrangedObjects];
NSUInteger index = [objects indexOfObjectPassingTest:^(id object, NSUInteger index, BOOL *stop) {
    if([[object valueForKey:@"userName"] isEqualToString:@"theNewUsername"]) {
        *stop = YES;
        return YES;
    }
    return NO;
}];
if(index == NSNotFound) {
    // user name available
} else {
    // user name taken by the object at "index" within "objects"
}