iPhone - Core Data Bool Values?

I hav开发者_如何学Goe a BOOL in my core data. I want the value to be null unless i set it to either YES, or NO. Is it possible to do this? in my code how can i check to see if the value has not been set yet or not?

In my nsmanagedObject i use NSNumber for my BOOL, in my core data mdoel i have BOOL.

CoreData allows you to set default values. A Bool can contain "YES", "NO" , "None". Setting the default value to "None" makes the returned value "null" which is exactly what i needed.

If you set Boolean as the attribute type in your model, that means it will be implemented using NSNumber.

I'm not sure why you'd want to return no value instead of NO, I'd actually recommend you don't. You'll have a much easier time with data integrity if you just set a false default value.

However, if you absolutely insist, you can just check if the attribute is nil or not to see if it was assigned a value yet.

The correct answer is yes, and there is real meaning to use 'yes' 'no' and a different 'none', which means the data is intact. In Core Data there is good reason sometimes to check against 'none', especially if you'd like to debug a database. In Core Data you can use:

[myManagedObject setValue: nil forKey:@"attributeOfMyManagedObject"];

and if check your SQL database behind your Core Data you will see 'NULL' if you use default settings.