How can I tell MagicalRecord to not use the file based Core Data but an in memory setup?

I followed this great article to get into Unit Testing regarding Core Data. The setup seems simple and involves just some view lines of code.

- (void)setUp;
{
    [MagicalRecord setDefaultModelWithClass:[self class]];
    [MagicalRecord setupCoreDataStackWithInMemoryStore];
}

- (void)tearDown;
{
    [MagicalRecord cleanUp];
}

- (void)testSomeCalculationOnMyEntity;
{
    NSNumber *count = [MyEntity MR_numberOfEntities];
    // STAssert([testEntity customCalculation] == expectedValue, @"expected a good calculation");
}

@end

The problem is, that each time I for example check for the amount of entities in the in memory set up of Core Data by calling [MyEntity MR_numberOfEntities] (like above), I get the amount of objects, which are stored in the file based setup which are a couple of thousand objects. How does this happen? I mean the second line in setUp indicates the in memory one, doesn't it? And this case should return 0 as the amount of objects stored.

Thanks for any suggestions!

Edit:

@casademora put me on the right track. The following works setup works fine for me now.

- (void)setUp;
{
    [MagicalRecord cleanUp]; // This solved the mystery.

    // I don't now why I had to remove this line, though.
    // [MagicalRecord setDefaultModelWithClass:[self class]];

    [MagicalRecord setupCoreDataStackWithInMemoryStore];
}

- (void)tearDown;
{
    [MagicalRecord cleanUp];
}

- (void)testSomeCalculationOnMyEntity;
{
    NSNumber *count = [MyEntity MR_numberOfEntities];
    // STAssert([testEntity customCalculation] == expectedValue, @"expected a good calculation");
}

@end