the default of attribute in coredata

I did versioning and I added a new attribute of type string

it will be empty at first I want to create a predicate to check for the empty attribute I tried

[fetchRequest setPredicate:[NSPredicate predicateWithFormat:
                            @"( SeriesStudyID == 开发者_JAVA技巧'""' )"  ]]; 

but it doesn't work

what's the default of this attribute and how to detect the empty

---

SeriesStudyID is NSString

if ([SeriesStudyID length]<=0) {

    NSLog(@"empty");
}

else {
    NSLog(@"not empty");
}