Core data one-to-many relationships - ios

Question

I've been searching around the internet for days looking for tutorials on how to work with two entities and adding values for the data and linking them. Here is how my app is set up:
I'm making an app that allows the user to create an athlete and within that athlete they can add multiple sporting events. I have two entities: Athletes and Events with a to-one relation from Athletes to Events and a to-many relationship inversely.
The issue I'm having is what code to write to add the name and opponent attribute values within the Events entitiy, all while making sure that that specific name and opponent match up to only one athlete. I've tried using Core Data Accessor Methods, as well as creating new NSManagedObjects for either entitity and adding the values for specific keys.
I've tried to follow the CoreDataRecipes sample code, as well as the common Core Data tutorials on the web. Can anyone help steer me in the right path with some basic method functions or other tutorials that helped you? Thanks.

Answer 1

You need to stop thinking in terms of RDBMS (relational databases) and start thinking in terms of managed object models. CoreData deals with managing objects and their associations. You can associate Athlete objects to Event objects (by the way I suggest using singular vs. plural for entity names i.e. Athlete and Event vs. Athletes and Events and use singular for to-one relationships and plural for to-many relationships). It's a preference thing but I refer to myself as an Athlete (object) not an Athletes (objects). Makes it all more readable and intuitive as well.
Assuming your entities look like this given how you described the relationships:
#interface Athlete : NSManagedObject
#property (nonatomic, retain) NSString * name;
//... a bunch more attributes
#property (nonatomic, retain) Event *event; // use singluar for relationship name too
//...
#end
#interface Event : NSManagedObject
#property (nonatomic, retain) NSString * eventName;
//... a bunch more attributes
#property (nonatomic, retain) NSSet *athletes; // use plural for relationship name
//...
#end
#implementation MyViewController
//... some method
// fetch the athletes (possibly present in table view or other mechanism for selection)
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:#"Athlete"];
NSArray *athletes = [managedObjectContext executeFetchRequest:fetchRequest error:nil];
// select the althletes (primary & opponent - hardcoded for example)
//...
NSArray *selectedAthletes = [NSArray arrayWithObjects: athletes[0], athletes[1], nil];
// create an event
Event *event = [NSEntityDescription
insertNewObjectForEntityForName:#"Event"
inManagedObjectContext:context];
// add the athletes
[event addAthletes:[NSSet setWithArray:selectedAthletes]];
//...
#end
Now you have an event with the 2 athletes. If you wanted to distinguish between the opponent and challenger then you could create 2 to-one relationships (from Event to Athlete) such as opponent and challenger and associate the Event to the Athlete via something like:
#interface Event : NSManagedObject
#property (nonatomic, retain) NSString * eventName;
//... a bunch more attributes
#property (nonatomic, retain) Athlete *opponent; // use singluar for relationship name
#property (nonatomic, retain) Athlete *challenger;
//...
#end
//...
event.opponent = athletes[0];
event.challenger = athletes[1];
//...