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

I wrote a couple of category methods to strip nulls from a JSON-generated dictionary or array prior to use:

@implementation NSMutableArray (StripNulls)

- (void)stripNullValues
{
    for (int i = [self count] - 1; i >= 0; i--)
    {
        id value = [self objectAtIndex:i];
        if (value == [NSNull null])
        {
            [self removeObjectAtIndex:i];
        }
        else if ([value isKindOfClass:[NSArray class]] ||
                 [value isKindOfClass:[NSDictionary class]])
        {
            if (![value respondsToSelector:@selector(setObject:forKey:)] &&
                ![value respondsToSelector:@selector(addObject:)])
            {
                value = [value mutableCopy];
                [self replaceObjectAtIndex:i withObject:value];
            }
            [value stripNullValues];
        }
    }
}

@end


@implementation NSMutableDictionary (StripNulls)

- (void)stripNullValues
{
    for (NSString *key in [self allKeys])
    {
        id value = [self objectForKey:key];
        if (value == [NSNull null])
        {
            [self removeObjectForKey:key];
        }
        else if ([value isKindOfClass:[NSArray class]] ||
                 [value isKindOfClass:[NSDictionary class]])
        {
            if (![value respondsToSelector:@selector(setObject:forKey:)] &&
                ![value respondsToSelector:@selector(addObject:)])
            {
                value = [value mutableCopy];
                [self setObject:value forKey:key];
            }
            [value stripNullValues];
        }
    }
}

@end

It would be nice if the standard JSON parsing libs had this behaviour by default - it's almost always preferable to omit null objects than to include them as NSNulls.

Answer 3

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

Answer 4

Another method is

-[NSObject setValuesForKeysWithDictionary:]

In this scenario you could do

[fight setValuesForKeysWithDictionary:dict];

In the header NSKeyValueCoding.h it defines that "Dictionary entries whose values are NSNull result in -setValue:nil forKey:key messages being sent to the receiver.

The only downside is you will have to transform any keys in the dictionary to keys that are in the receiver. i.e.

dict[@"winnerID"] = dict[@"winner"];
[dict removeObjectForKey:@"winner"];

Answer 5

I was stuck with the same problem, found this post, did it in a slightly different way.Using category only though -

Make a new category file for "NSDictionary" and add this one method -

@implementation NSDictionary (SuperExtras)

- (id)objectForKey_NoNSNULL:(id)aKey
{
    id result = [self objectForKey:aKey];

if(result==[NSNull null])
{
    return nil;
}

return result;

}

@end

Later on to use it in code, for properties that can have NSNULL in them just use it this way -

newUser.email = [loopdict objectForKey_NoNSNULL:@"email"];

Thats it