Error with NSJSONSerialization - Invalid type in JSON write (Menu)

I have an App using core data with 3 entities with very similar attributes. The relationship is such as:

Branch ->> Menu ->> Category ->> FoodItem

Each entity has an associated class: example

I am trying to generate JSON representation of the data in sqlite database.

//gets a single menu record which has some categories and each of these have some food items
id obj = [NSArray arrayWithObject:[[DataStore singleton] getHomeMenu]]; 
                 
NSError *err;
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:obj options:NSJSONWritingPrettyPrinted error:&err];
                 
NSLog(@"JSON = %@", [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding]);

But instead of JSON, i get a SIGABRT error.

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid type in JSON write (Menu)'

Any ideas how to fix it or how to make the entity classes (Branch, Menu etc) JSON serialization compatible?

That's because your "Menu" class is not serializable in JSON. Bascially the language doesn't know how your object should be represented in JSON (which fields to include, how to represent references to other objects...)

From the NSJSONSerialization Class Reference

> An object that may be converted to JSON must have the following > properties: > > - The top level object is an NSArray or NSDictionary. > - All objects are instances of NSString, NSNumber, NSArray, NSDictionary, or NSNull. > - All dictionary keys are instances of NSString. > - Numbers are not NaN or infinity.

This means that the language knows how to serialize dictionaries. So a simple way to get a JSON representation from your menu is to provide a Dictionary representation of your Menu instances, which you will then serialize into JSON:

- (NSDictionary *)dictionaryFromMenu:(Menu)menu {
    [NSDictionary dictionaryWithObjectsAndKeys:[menu.dateUpdated description],@"dateUpdated",
    menu.categoryId, @"categoryId",
    //... add all the Menu properties you want to include here
    nil];
}

And you could will use it like this :

NSDictionary *menuDictionary = [self dictionaryFromMenu:[[DataStore singleton] getHomeMenu]]; 

NSError *err;
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:menuDictionary options:NSJSONWritingPrettyPrinted error:&err];

NSLog(@"JSON = %@", [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding]);

There is a class method isValidJSONObject on NSJSONSerialization that tells you if a object can be serialised. As Julien pointed out you probably have to convert your object to a NSDictionary. NSManagedModel provides some handy methods to get all your attributes for your entity. So you could create a category for NSManagedObject that has a method to convert it over to a NSDictionary. This way you don't have to write a toDictionary method for each entity you want to convert to a dictionary.

@implementation NSManagedObject (JSON)

- (NSDictionary *)toDictionary
{
    NSArray *attributes = [[self.entity attributesByName] allKeys];
    NSDictionary *dict = [self dictionaryWithValuesForKeys:attributes];
    return dict;
}

You can use + isValidJSONObject: method of NSJSONSerialization class. If it is not valid, you can use - initWithData:encoding: method of NSString.

- (NSString *)prettyPrintedJson:(id)jsonObject
{
    NSData *jsonData;
    
    if ([NSJSONSerialization isValidJSONObject:jsonObject]) {
        NSError *error;
        jsonData = [NSJSONSerialization dataWithJSONObject:jsonObject
                                                           options:NSJSONWritingPrettyPrinted
                                                             error:&error];
        
        if (error) {
            return nil;
        }
    } else {
        jsonData = jsonObject;
    }
    
    return [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
}

I had the key switched with the value : @{value :@"key"} It should be @{@"key":value}