Core data with multiply relationships

Question

I'm trying store and retrieve data using core data. I'm kind-of okay with working with dual entity's but currently trying to access an (PRC) entity which is a two levels deep.

Currently my model consists of a Fpga entity, a Session entity and an PRC entity. The Fpga entity has a one-to-many relationship to a Session entity. The Session entity has an one to one relationship to a PRC entity. The PRC entity consists of two attributes prc0 and prc1.

I'm currently populating the model from:

Fpga *newEntry = [NSEntityDescription insertNewObjectForEntityForName:@"Fpga" inManagedObjectContext:self.managedObjectContext];

newEntry.name = self.fpgaTextField.text;

Session *session0 = [NSEntityDescription insertNewObjectForEntityForName:@"Session" inManagedObjectContext:self.managedObjectContext];
session0.type = self.s0TextField.text;

//--------
session0.prc.prc0 = [NSNumber numberWithInt:99];
session0.prc.prc1 = [NSNumber numberWithInt:113];
//--------

Session *session1 = [NSEntityDescription insertNewObjectForEntityForName:@"Session" inManagedObjectContext:self.managedObjectContext];
session1.type = self.s1TextField.text;

//------
session1.prc.prc0 = [NSNumber numberWithInt:[ self.p0TextField.text intValue]];;
session1.prc.prc1 = [NSNumber numberWithInt:[ self.p1TextField.text intValue]];;
//------

newEntry.session = [NSSet setWithObjects:session0, session1, nil];

And trying retrieving the PRC data like (only one object in the database for the demo):

    PRC *prc0 = (PRC*)[[[fpgaObj.session allObjects] objectAtIndex:0] prc];
    self.p0TextField.text = [NSString stringWithFormat:@"%@", prc0.prc0];

I'm always returning null but all other Fpga and Session attributes are fine. Obviously somethings wrong... but not sure what..

Thanks in advance

Answer 1

You do not create the PRC objects, therefore session0.prc and session1.prc are nil .

For example:

PRC *prc = [NSEntityDescription insertNewObjectForEntityForName:@"PRC" inManagedObjectContext:self.managedObjectContext];
prc.prc0 = [NSNumber numberWithInt:99];
prc.prc1 = [NSNumber numberWithInt:113];
session0.prc = prc;

Answer 2

Before you can do session0.prc.prc0 = ... you need to create an instance of the PRC entity. You would usually create and configure this instance and then set it as the contents of the relationship ( session0.prc = ... ). At the moment you are trying to set the numbers into a non-existent instance.