Question

I have a CoreData model with 2 objects. The first object (Images) contains a list of image names. The second object (Book) has a relationship with a field from the Images object called imageId. The data type is set to int16 and is marked optional with a default value of 0, as not every book will have an image.

I request a JSON representation of the Book object from a WCF service. One of the book objects returned has “ImageId” : null as its value. So when converting the JSON object into a managed object in Objective-C I get the following error message:

Unacceptable type of value for attribute: property = “imageId”; desired type = NSNumber; given type = NSNull; value =

How do you handle this conversion? I thought of checking for null and then setting the value to 0 but this doesn’t seem correct.

Many thanks.

*UPDATE *
In the process of implementing Daniels solution I discovered that other fields could also be returned from the service as null. Is it therefore possible to modify the NSDictionary category to enable it to traverse itself and replace any instance of with nil? I am using jsonKit as my JSON parser and it seems to return the parsed result as a NSMutableDictionary with NSDictionary objects representing my JSON objects. I have spent much of the day trying to modify my objects but as they are returned as NSDictionaries they are immutable. I could simply use Daniels solution on every field that needed checking for however if the service changed and another field was returned as then this would break my app. Alternatively I could implement Daniels solution on every field just incase it was ever but not sure if thats a good solution especially for performance.

Answer 1

Some JSON libraries let you set an option so that JSON nulls are simply omitted from the dictionary as opposed to using NSNull. If you do that, then it will work because [theDict objectForKey:@"imageID"] will return nil.

But other than that, the right thing to do is look out for NSNull explicitly. Making an NSDictionary category is a handy way to do it:

@implementation NSDictionary (DDNullGetter)

- (id)nonNullObjectForKey:(id)key
{
    id val = [self objectForKey:key];
    return (val == [NSNull null] ? nil : val);
}

@end