objective c - set relationship (core data) - ios

Question

I'm very new to core data and the project that I'm currently working on involves using core data for local data storage.
Below is the structure of my entities.
My app functions as follows. When the user first launches the app, there are presented with a login screen. New users can register. When a user tries to register, they have to enter a unique key that has already been provided to them, prior to the registration. This unique key is validated in the Facility table. If the key exists, in the db, then I will enter the user data into the database and will create a relationship between the Assessor and the Facilitythat they just entered the code for.
My question is, how would I create a relationship between a new Assessor and an existing Facility?
I'm thinking, I should fetch the required Facility object from core data and then use [assessorsetValue:facilityObjectforKey:#"facility"] to set the object.
Can anyone help me understand how to fetch the facilityObject from the Facility table in core data before assigning it to the Assessor?
Thanks in advance.
EDIT:
For example:
This is how my Facility table will look like.
code | name
H6DJ | Computing
So if the user (John Doe) enters H6DJ as the registration code, I check the Facility table to see if it exists. so in this case, I would like to create a relationship between the new user (John Doe) and the Computing facility.

Answer 1

The Core Data Programming Guide provides lots of sample code.
Your idea about setting the facility property of the new assessor entity is correct - you just need to fetch the facility first - presumably you would need to do this anyway to validate the facility code that was entered.
NSManagedObject *newAssessor = [NSEntityDescription
insertNewObjectForEntityForName:#"Assessor"
inManagedObjectContext:context];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity =
[NSEntityDescription entityForName:#"Facility"
inManagedObjectContext:managedObjectContext];
[request setEntity:entity];
NSPredicate *predicate =
[NSPredicate predicateWithFormat:#"code == %#", targetCode];
[request setPredicate:predicate];
NSError *error;
NSArray *array = [managedObjectContext executeFetchRequest:request error:&error];
if (array != nil) {
NSUInteger count = array.count;
if (count == 1) {
NSManagedObject *facility=array[0];
if (facility["assessor"]==nil) {
newAssessor["facility"]=facility;
// Set up rest of assessor attributes before saving
} else {
// Error - facility already assigned to assessor ?
}
} else if (count == 0) {
// Code not found/valid - do something
} else {
// More than 1 matching facility - data error
}
}
else {
// Deal with error.
}