How to unit test my models now that I am using Core Data?

I have been developing an iphone application using a domain model, and have put off the persistence aspect of the app until now. Core Data looks like a really good solution since I already have a well defined model but I am running into a snag with my existing unit tests.

Here is simple example of what I have now:

- (void)test_full_name_returns_correct_string {
    Patient *patient = [[Patient alloc] init];  
    patient.firstName = @"charlie";
    patient.lastName = @"chaplin";
    STAssertTrue([[patient fullName] isEqualToString:@"charlie chaplin"], @"should have matched full name");
}  

How can I make this work once my Patient object extends from NSManagedObject and uses @dynamic for the firstName and lastName properties?

Has anyone else run into this type of this with Core Data? Thanks.

You need to build a Core Data stack, either within each method or in -setUp and then tear it down. Using an NSInMemoryPersistentStore will keep things fast and in-memory for your unit tests. Add a @property (nonatomic,retain) NSManagedObjectContext *moc to your TestCase subclass. Then:

- (void)setUp {
  NSManagedObjectModel *mom = [NSManagedObjectModel mergedModelFromBundles:[NSArray arrayWithObject:bundleContainingXCDataModel]];
  NSPersistentStoreCoordinator *psc = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:mom];
  STAssertTrue([psc addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:NULL] ? YES : NO, @"Should be able to add in-memory store");    
  self.moc = [[NSManagedObjectContext alloc] init];
  self.moc.persistentStoreCoordinator = psc;

  [mom release];
  [psc release];
  
}

- (void)tearDown {
  self.moc = nil;
}

Your test method then looks like:

- (void)test_full_name_returns_correct_string {
    Patient *patient = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:self.moc];

    patient.firstName = @"charlie";
    patient.lastName = @"chaplin";
    STAssertTrue([[patient fullName] isEqualToString:@"charlie chaplin"], @"should have matched full name");
}

assuming your entity is named Person. There was a memory leak in your version of the method, by the way; patient should be -release'd in the non-Core Data version (insertNewObjectForEntityForName:managedObjectContext: returns an autoreleased instance).

I used the answer above by Barry Wark, but I had to do some modifications to make it work with the current projects Xcode 5, iOS 7.

The property stayed the same:

@interface SIDataTest : XCTestCase
    @property (nonatomic, retain) NSManagedObjectContext *moc;
@end

The setup had to actually had to change first of all to not release and secondly to provide a model URL.

- (void)setUp
{
    [super setUp];
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"SimpleInvoice" withExtension:@"momd"];
    NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    NSPersistentStoreCoordinator *psc = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:mom];
    XCTAssertTrue([psc addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:NULL] ? YES : NO, @"Should be able to add in-memory store");
    self.moc = [[NSManagedObjectContext alloc] init];
    self.moc.persistentStoreCoordinator = psc;
}

Here is the example test case:

- (void)testCreateNew
{
    Invoice *newInvoice = [NSEntityDescription insertNewObjectForEntityForName:@"Invoice" inManagedObjectContext:self.moc];
    newInvoice.dueDate = [NSDate date];
    NSString* title = [[NSString alloc] initWithFormat:@"Invoice %@", @112];
    newInvoice.title = title;
    
    // Save the context.
    NSError *error = nil;
    if (![self.moc save:&error]) {
        // Replace this implementation with code to handle the error appropriately.
        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
        XCTFail(@"Error saving in \"%s\" : %@, %@", __PRETTY_FUNCTION__, error, [error userInfo]);
    }
    XCTAssertFalse(self.moc.hasChanges,"All the changes should be saved");
}