How do you manage and use “Many to many” core data relationships?

Question

I\'m creating an app and trying to use core data, because it seems like it\'s the objective-C approved way to create a data storage system. The use case I have involves \"ma

Answer 1

Let's say you have two entities in your database, employees and managers. They would look something like this in the code:

@interface Employee :  NSManagedObject  
{

}

@property (nonatomic, retain) NSNumber * id;
@property (nonatomic, retain) NSSet* managers;

@interface Manager :  NSManagedObject  
{

}

@property (nonatomic, retain) NSNumber * id;
@property (nonatomic, retain) NSSet* employees;

So let's say you wanted to retrieve all the employees for manager number 1, you would just have to get the manager object from the database:

NSNumber *managerID = [NSNumber numberWithInt:1];
NSEntityDescription * entityDescription = [NSEntityDescription entityForName:@"Manager"                                                       inManagedObjectContext:managedObjectContext]; 
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];  
[request setEntity:entityDescription];  
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%K == %d", @"id", managerID];   
[request setPredicate:predicate];
NSArray *array = [managedObjectContext executeFetchRequest:request error:&error];
Manager *manager = [array objectAtIndex:0];

and then you would have all its employees in the employees property:

NSSet *allHisEmployees = manager.employees;

It would be exactly the same to retrieve the managers for a given employee.

I hope it clears things up