changing [ret description] to ret does not work as expected

hash or isEqual issue

NSMutableArray *ivarPrevious = [NSMutableArray arrayWithCapacity:1];

-(void)foo
{
    NSMutableArray *newivarPrevious = [NSMutableArray arrayWithCapacity:1];
    for (...){           
        NSMutableDictionary *ret = [NSMutableDictionary dictionaryWithCapacity:1];
        [ret setObject:foo forKey:@"foo"];
        [ret setObject:bar forKey:@"bar"];
        [newivarPrevious addObject:[ret description]];                
        if (![ivarPrevious containsObject:[ret description]]) { 
            //new
        }
    }
    [ivarPrevious removeAllObjects];
    [ivarPrevious setArray:newivarPrevious];
}