如何在带有实体框架4.0的MVC4中使用ViewModel模拟方法

我想测试以下方法：

public ActionResult Index()
{
    var transactions = db.Transactions.Include(t => t.User)
                            .GroupBy(t => t.UserId)
                            .Select(group => new TransactionViewModel
                            {
                                User = group.FirstOrDefault().User.FullName,
                                UserId = group.FirstOrDefault().UserId,
                                Total = (group.Sum(t => t.TransactionAmount))
                            });

    // Show lowest balance first
    return View(transactions.ToList());
}

该Transaction模型在此处具有的列表Orders，具有的外键User和更多属性，请参见：

public class Transaction
{
    public int TransactionId { get; set; }
    public DateTime Date { get; set; }
    public int UserId { get; set; }
    public List<Order> Orders { get; set; }
    public decimal TransactionAmount { get; set; }
    public virtual User User { get; set; }
}

该TransactionViewModel如下外观：

public class TransactionViewModel
{
    public string User { get; set; }
    public int UserId { get; set; }
    public decimal Total { get; set; }
}

并用于计算Total属于用户的不同交易的金额。

为了测试此方法，我在以下安装程序中FakeDbSet使用FakeContext，并使用了（两者都可在其他控制器的测试中使用）：

[TestClass]
public class TransactionControllerTest
{
    TransactionController trController;

    [TestInitialize]
    public void TransactionControllerTestInitialize()
    {
        // Arrange 
        var memoryTransactionItems = new FakeDbSet<Transaction>
        {
           new Transaction {
               Date = DateTime.Today,
               TransactionAmount = 5.10M,
               UserId = 1,
               Orders = new List<Order>{
                    // Categorie 2 and confirmed
                    new Order { OrderId = 2, 
                                UnitPrice = 2.00M, 
                                Quantity = 1, 
                                Date = DateTime.Today, 
                                IsConfirmed = true, 
                                User = new User { 
                                    Name = "Kees", 
                                    FullName="Kees Piet", 
                                    Email = "[email protected]", 
                                    isAvailable = true, 
                                    UserId = 1 
                                }, 
                                Product = new Product {
                                    Category = new Category {
                                        CategoryId = 2, 
                                        Name = "Categorie2"
                                    }, 
                                    Name = "Testproduct2",
                                    Price = 2.00M,
                                    Visible = true
                                }
                    },
                    // Categorie 2 and confirmed
                    new Order { OrderId = 2, 
                                UnitPrice = 1.00M, 
                                Quantity = 1, 
                                Date = DateTime.Today, 
                                IsConfirmed = true, 
                                User = new User { 
                                    Name = "Jan", 
                                    FullName="Jan Piet", 
                                    Email = "[email protected]", 
                                    isAvailable = true, 
                                    UserId = 2 
                                }, 
                                Product = new Product {
                                    Category = new Category {
                                        CategoryId = 2, 
                                        Name = "Categorie2"
                                    }, 
                                    Name = "Testproduct2",
                                    Price = 3.10M,
                                    Visible = true
                                }
                    }
               }
           }
        };


        // Create mock units of work
        var mockData = new Mock<FakeContext>();
        mockData.Setup(m => m.Transactions).Returns(memoryTransactionItems);

        // Setup controller
        trController = new TransactionController(mockData.Object);
    }

    [TestMethod]
    public void TestTransactionIndex()
    {
        // Invoke
        var viewResult = trController.Index() as ViewResult;
        var transactionsFromView = (IEnumerable<TransactionViewModel>)viewResult.Model;

        // Assert
        Assert.AreEqual(1, transactionsFromView.Count(),
            "The amount of transactions added to the Index View should be 1.");
    }
}

当我运行时，TestTransactionIndex出现以下错误：

测试名称：TestTransactionIndex测试结果：测试失败持续时间：0：00：30.6276475

结果消息：测试方法Tests.Controllers.TransactionControllerTest.TestTransactionIndex引发异常：System.NullReferenceException：对象引用未设置为对象的实例。结果StackTrace：位于lambda_method（Closure，IGrouping 2 ) at System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext（）
位于System.Collections.Generic.List 1..ctor(IEnumerable1集合）
位于System.Linq.Enumerable.ToList [TSource]（IEnumerable`1源）位于Controllers.TransactionController.Index（）

我发现这很奇怪，因为我以正确的方式设置了模拟单元。希望有人能解释我如何正确地将FakeDbSet<Transaction>视图发送给视图而不获取视图NullReferenceException。

/编辑根据要求，这里是构造函数TransactionController：

private IContext _context;

public TransactionController()
{
    _context = new Context();
}

public TransactionController(IContext context)
{
    _context = context;
}