NSNull handling for NSManagedObject properties values

Question

I'm setting values for properties of my NSManagedObject, these values are coming from a NSDictionary properly serialized from a JSON file. My problem is, that, when some value is [NSNull null], I can't assign directly to the property:

    fight.winnerID = [dict objectForKey:@"winner"]; 

this throws a NSInvalidArgumentException

"winnerID"; desired type = NSString; given type = NSNull; value = <null>; 

I could easily check the value for [NSNull null] and assign nil instead:

fight.winnerID = [dict objectForKey:@"winner"] == [NSNull null] ? nil : [dict objectForKey:@"winner"]; 

But I think this is not elegant and gets messy with lots of properties to set.

Also, this gets harder when dealing with NSNumber properties:

fight.round = [NSNumber numberWithUnsignedInteger:[[dict valueForKey:@"round"] unsignedIntegerValue]] 

The NSInvalidArgumentException is now:

[NSNull unsignedIntegerValue]: unrecognized selector sent to instance 

In this case I have to treat [dict valueForKey:@"round"] before making an NSUInteger value of it. And the one line solution is gone.

I tried making a @try @catch block, but as soon as the first value is caught, it jumps the whole @try block and the next properties are ignored.

Is there a better way to handle [NSNull null] or perhaps make this entirely different but easier?

Answer 1

It might be a little easier if you wrap this in a macro:

#define NULL_TO_NIL(obj) ({ __typeof__ (obj) __obj = (obj); __obj == [NSNull null] ? nil : obj; }) 

Then you can write things like

fight.winnerID = NULL_TO_NIL([dict objectForKey:@"winner"]); 

Alternatively you can pre-process your dictionary and replace all NSNulls with nil before even trying to stuff it into your managed object.

Answer 2

Ok, I've just woke up this morning with a good solution. What about this:

Serialize the JSON using the option to receive Mutable Arrays and Dictionaries:

NSMutableDictionary *rootDict = [NSJSONSerialization JSONObjectWithData:_receivedData options:NSJSONReadingMutableContainers error:&error]; ... 

Get a set of keys that have [NSNull null] values from the leafDict:

NSSet *nullSet = [leafDict keysOfEntriesWithOptions:NSEnumerationConcurrent passingTest:^BOOL(id key, id obj, BOOL *stop) {     return [obj isEqual:[NSNull null]] ? YES : NO; }]; 

Remove the filtered properties from your Mutable leafDict:

[leafDict removeObjectsForKeys:[nullSet allObjects]]; 

Now when you call fight.winnerID = [dict objectForKey:@"winner"]; winnerID is automatically going to be (null) or nil as opposed to <null> or [NSNull null].

Not relative to this, but I also noticed that it is better to use a NSNumberFormatter when parsing strings to NSNumber, the way I was doing was getting integerValue from a nil string, this gives me an undesired NSNumber of 0, when I actually wanted it to be nil.

Before:

// when [leafDict valueForKey:@"round"] == nil fight.round = [NSNumber numberWithInteger:[[leafDict valueForKey:@"round"] integerValue]] // Result: fight.round = 0 

After:

__autoreleasing NSNumberFormatter* numberFormatter = [[NSNumberFormatter alloc] init]; fight.round = [numberFormatter numberFromString:[leafDict valueForKey:@"round"]];     // Result: fight.round = nil