CoreDataExample.swift

import CoreData
import UIKit

class ViewController: UIViewController {

    var appDelegate : AppDelegate?
    var managedContext : NSManagedObjectContext?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        appDelegate = UIApplication.sharedApplication().delegate as? AppDelegate
        managedContext = appDelegate?.managedObjectContext
        
        let companies = fetchCompanies()
        
        if companies.count == 0 {
            createACompany()
        } else {
            for company in companies {
                let employees = getEmployees(forCompany: company)
                
                for employee in employees { print(employee.name) }
            }
        }
        
        
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func getEmployees(forCompany company : Company) -> [Employee] {
        
        guard let employeeSet = company.employees, let employees = employeeSet.allObjects as? [Employee] else {
            return []
        }
        
        return employees
        
    }
    
    func createACompany() {
        
        guard let moc = managedContext else { return }
        
        guard let company = NSEntityDescription.insertNewObjectForEntityForName("Company", inManagedObjectContext: moc) as? Company else {
            return
        }
        company.name = "Apple"
        
        guard let bob = NSEntityDescription.insertNewObjectForEntityForName("Employee", inManagedObjectContext: moc) as? Employee else {
            return
        }
        bob.name = "Bob"
        bob.company = company
        
        do {
            try moc.save()
        } catch {
            print("core data save error : \(error)")
        }
        
        moc.refreshAllObjects()
        
    }
    
    func fetchCompanies() -> [Company] {
        
        guard let moc = managedContext else { return [] }
        let request = NSFetchRequest(entityName: "Company")
        request.returnsObjectsAsFaults = true
        
        do {
            let results = try moc.executeFetchRequest(request)
            
            guard let companies = results as? [Company] else {
                return []
            }
            
            return companies
            
        }catch let error as NSError{
            print("core data fetch error: \(error)")
            return []
        }
        
    }
}