Core Data and OCMock

Models backed by a data persistence layer are too important to simply punt on proper testing. The rest of the app expects the data models to work as designed; data persistence and all. This article presents a solution to unit testing Core Data models using OCMock and Magical Record.

The straight mocking problem

If you’re familiar with OCMock, this code may look familiar. If you’ve never used OCmock before, take a look at this tutorial, and the OCMock site for additional resources. Anyway, let’s pretend we have a simple NSManaged object we’re innocently trying to test:

// data model for Person
// name -- NSString
// lunchOrder -- NSString
// cost -- int16

NSEntityDescription *pDesc = [NSEntityDescription entityForName:@"Person"
inManagedObjectContext:[NSManagedObjectContext MR_defaultContext]];
Person *realPersonModel = [[Person alloc] initWithEntity:pDesc
insertIntoManagedObjectContext:[NSManagedObjectContext MR_defaultContext]];

id personMock = [OCMockObject partialMockForObject: realPersonModel];

// argh! selector not defined, what's going on?
// OCMockObject wrecked Core Data @dynamic selectors

XCTestAssertNil(, @"person not nil");

The code above creates a new Person object and inserts it into the default NSManagedObjectContext using helpers provided by Magical Record. A partial mock of ‘Person’ object is created with OCMock. However, whenever a property of the NSManagedObject is accessed, we receive an odd ‘unknown selector’ error.

@Dynamic properties

Core Data implements property accesors through @dynamic properties. Any property marked @dynamic means the class will figure out howto respond to the selector at runtime. In the case of Core Data, the NSManagedObject provides accessors for us to read these fields from the persistent store. Unlike @property, an automatic ivar is not created when using @dynamic. When an NSManagedObject is passed to OCMock mock object or partialMockForObject the @dynamic selectors aren’t found and results in an error.

Never, ever mock Core Data

Don’t directly mock Core Data. There is too much proprietary weird stuff going on to trust the test completely. If this feels dirty to you, another option is to abstract any business operations out of your NSManagedObjects into a service layer which can be properly mocked.

In memory store

When testing with XCTest, the + setUp and + tearDown methods create a new in-memory Core Data stack and tear it down once the tests have completed. I find this provides the speed and test isolation necessary to ‘trust’ the tests between classes.

+ (void)setUp {
[MagicalRecord setupCoreDataStackWithInMemoryStore];

+ (void)tearDown {
[MagicalRecord cleanUp];

Stub without wrecking @dynamic properties

In this example we don’t be using a Core Data mock directly, but will still make use of OCMock stubs.

NSEntityDescription *pDesc = [NSEntityDescription entityForName:@"Person" 
inManagedObjectContext:[NSManagedObjectContext MR_defaultContext]];

// dont pass this model to OCMock, but use this model within assertions or pass to other components.
Person *realPersonModel = [[Person alloc] initWithEntity:pDesc
insertIntoManagedObjectContext:[NSManagedObjectContext MR_defaultContext]];

// OCMock will wreak out @dynamic properties, so make a new model for mock partial
Person *modelForMocking = [[Person alloc] initWithEntity:pDesc
insertIntoManagedObjectContext:[NSManagedObjectContext MR_defaultContext]];

// create our stubs on this mock
id groupModelMock = [OCMockObject partialMockForObject:modelForMocking];
[[[groupMock stub] andReturn:@"ZachDude"] name];

// oddly enough, now calling will return @"ZachDude".

Programmer discovers howto use OCMock with Core Data using this one weird trick!

Creating two NSManagedObjects, one for use within assertions and another to feed into OCMock for stubbing, etc works because of this reason. (taken from OCMock docs)

Partial Mocks

Creates a mock object that can be used in the same way as anObject. When a method that is not stubbed is invoked it will be forwarded to anObject. When a stubbed method is invoked using a reference to anObject, rather than the mock, it will still be handled by the mock.

In laymens terms, this means by passing modelForMocking to partialForMock:, the other Person model, realPersonModel is also affected. Except now that realPersonModel can finally resolve it’s @dynamic selectors, we can contiue to test in comfort.