Unit testing 单元测试核心数据和NSExpressionDescription方法

如何对以下代码进行单元测试

NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
NSString *sumOfTotalPaidBySelf = @"sumOfTotalPaidBySelf";
expressionDescription.name = sumOfTotalPaidBySelf;
expressionDescription.expression = [NSExpression expressionForKeyPath:@"@sum.moneyInMainCurrency"];
expressionDescription.expressionResultType = NSDoubleAttributeType;

NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"MCPayment"];
fetchRequest.resultType = NSDictionaryResultType;
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"payingPerson = %@", self];
fetchRequest.propertiesToFetch = @[expressionDescription];

NSError *error = nil;
NSArray *fetchResult = [[self managedObjectContext] executeFetchRequest:fetchRequest error:&error];
if (error) {
    NSLog(@"%@: error fetching: %@", self, error);
    return nil;
}

return [[fetchResult firstObject] objectForKey:sumOfTotalPaidBySelf];

到目前为止，我学到的是，当查询已包含数据的数据库时，该代码应该可以工作，该数据存储在磁盘上。但是，对于单元测试，我使用了NSInMemoryStoreType的persistentStoreType，它在安装过程中预先填充了一些数据，这些数据仍然驻留在managedObjectContext中

这是我的单元测试

MCSharedBill *tonightsBill = [MCSharedBill addSharedBillToContext:_context];
MCPerson *fred = [tonightsBill addPerson];
fred.firstName = @"Fred";
MCPerson *anna = [tonightsBill addPerson];
anna.firstName = @"Anna";
MCPayment *drinks = [tonightsBill addPayment];
drinks.payingPerson = fred;
drinks.money = @(10);
drinks.descriptionOfPayment = @"coffee";
NSNumber *totalPaidByFred = fred.totalSumPaid;
//    NSError *error = nil;
//    [_context save:&error]; 
//    XCTAssertFalse(error);
XCTAssertEqualWithAccuracy(@(10).doubleValue, totalPaidByFred.doubleValue, 0.001);

任何帮助都将不胜感激

干杯