NSInternalInconsistencyException for NSMutableArray - ios

Question

I'm trying to add objects to an NSMutableArray but it keeps giving me this error.:
NSInternalInconsistencyException', reason: '-[__NSCFArray insertObject:atIndex:]: mutating method sent to immutable object
I have researched this problem, and I'm not doing anything wrong that past people have done, so I have no idea what's wrong. Here is my code:
Group.h
#property (strong, nonatomic) NSString *custom_desc;
#property (strong, nonatomic) NSMutableArray *attributes; //I define the array as mutable
Group.m
#import "Group.h"
#implementation Group
-(id)init
{
self = [super init];
if(self)
{
//do your object initialization here
self.attributes = [NSMutableArray array]; //I initialize the array to be a NSMutableArray
}
return self;
}
#end
GroupBuilder.m
#import "GroupBuilder.h"
#import "Group.h"
#implementation GroupBuilder
+ (NSArray *)groupsFromJSON:(NSData *)objectNotation error:(NSError **)error
{
NSError *localError = nil;
NSDictionary *parsedObject = [NSJSONSerialization JSONObjectWithData:objectNotation options:0 error:&localError];
if (localError != nil) {
*error = localError;
return nil;
}
NSMutableArray *groups = [[NSMutableArray alloc] init];
NSDictionary *results = [parsedObject objectForKey:#"result"];
NSArray *items = results[#"items" ];
for (NSDictionary *groupDic in items) {
Group *group = [[Group alloc] init];
for (NSString *key in groupDic) {
if ([group respondsToSelector:NSSelectorFromString(key)]) {
[group setValue:[groupDic valueForKey:key] forKey:key];
}
}
[groups addObject:group];
}
for(NSInteger i = 0; i < items.count; i++) {
//NSLog(#"%#", [[items objectAtIndex:i] objectForKey:#"attributes"]);
NSMutableArray *att = [[items objectAtIndex:i] objectForKey:#"attributes"]; //this returns a NSArray object understandable
Group *g = [groups objectAtIndex:i];
[g.attributes addObjectsFromArray:[att mutableCopy]]; //I use mutable copy here so that i'm adding objects from a NSMutableArray and not an NSArray
}
return groups;
}
#end

Answer 1

Use options:NSJSONReadingMutableContainers on your NSJSONSerialization call.
Then all the dictionaries and arrays it creates will be mutable.

Answer 2

According to the error message you are trying to insert an object into an instance of NSArray, not NSMutableArray.
I think it is here:
NSMutableArray *att = [[items objectAtIndex:i] objectForKey:#"attrib`enter code here`utes"]; //this returns a NSArray object understandable
Items is fetched from JSON and therefore not mutable. You can configure JSONSerialization in a way that it creates mutable objects, but how exactly I don't know out of the top of my head. Check the references on how to do that or make a mutable copy:
NSMutableArray *att = [[items objectAtIndex:i] objectForKey:#"attributes"] mutableCopy];

Answer 3

Next try, considering your replies to the first attempt:
#import "Group.h"
#implementation Group
-(NSMutableArray*)attributes
{
return [[super attributes] mutableCopy];
}
#end