How to create a nested json object - objective-c

Question

"drive"=>{"admins_attributes"=>{"0"=>{"latitude"=>"42.2349343", "longitude"=>"-71.1133432"}, "1"=>{"latitude"=>"42.2349343", "longitude"=>"-71.1133432"}}}
Im trying to collect a bunch of latitudes and longitudes in objective c and save them to rails backend and using nested attributes but i can't figure out how to form this object like this in objective c. Each latitude and longitude has its own index.
NSDictionary *position = [[NSDictionary alloc] initWithObjectsAndKeys:
latitude, #"latitude",
longitude, #"longitude",
city, #"city",
state, #"state",
zipcode, #"zipcode",
carrName, #"carrier",
[NSNumber numberWithInt:signalStrength], #"signalStrength",
signalType, #"signalType",nil];
NSArray *positionArray = [NSArray arrayWithObjects:position, nil];
[positions addObject:positionArray];
NSError *error;
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
NSURL *url = [NSURL URLWithString:#"http://localhost:3001/api/v1/drives"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
[request addValue:#"application/json" forHTTPHeaderField:#"Content-Type"];
[request addValue:#"application/json" forHTTPHeaderField:#"Accept"];
[request setHTTPMethod:#"POST"];
NSDictionary *parameters = [[NSDictionary alloc] initWithObjectsAndKeys:positions, #"positions_attributes", nil];
NSData *postData = [NSJSONSerialization dataWithJSONObject:parameters options:NSJSONWritingPrettyPrinted error:&error];
[request setHTTPBody:postData];
NSLog(#"JSON = %#", [[NSString alloc] initWithData:postData encoding:NSUTF8StringEncoding]);
NSURLSessionDataTask *postDataTask = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSDictionary* json = [NSJSONSerialization JSONObjectWithData:data
options:kNilOptions
error:&error];
}];
[postDataTask resume];