未存储枚举和实体之间的多对多关系

Question

我正在开发一个实体框架代码优先项目，其中我有多对多关系：

服务提供者可以有很多服务类型，服务类型可以有很多服务提供者。

服务只有一种服务类型。

服务类型是一个枚举：

public enum ServiceTypeEnum
{
    Ambulance = 1,
    [Display(Name = "Cash Advance")]
    CashAdvance = 2,
    Hospitalization = 3,
    Hotel = 4,
    [Display(Name = "House Call")]
    HouseCall = 5,
    [Display(Name = "Medical Escort")]
    MedicalEscort = 6,
    Transfer = 7,
    Repatriation = 8
}

服务提供商

public partial class ServiceProvider
{
    public ServiceProvider()
    {
        ServiceTypes = new HashSet<ServiceTypeEnum>();
    }

    [Key]
    public int ServiceProviderID { get; set; }

    [Required]
    [StringLength(100)]
    public string Title { get; set; }

    // This is OK for a single Service 
    //public virtual ServiceTypeEnum ServiceType { get; set; }

    // I added this so that Service Providers can have multiple Service Types
    public ICollection<ServiceTypeEnum> ServiceTypes { get; set; }

    public IEnumerable<Service> Services { get; set; } 
}

和ServiceProviderViewModel：

public class ServiceProviderViewModel
{
    public class CreateModel
    {
        public int ServiceProviderID { get; set; }

        [Required]
        [StringLength(100)]
        [Display(Name = "Title")]
        public string Title { get; set; }

        public IEnumerable<KeyValuePair<string, int>> AllServiceTypes { get; set; }
        public string[] SelectedServiceTypes { get; set; }

        //public ServiceTypeEnum ServiceType { get; set; }
    }

    public class EditModel
    {
        ...
    }
}

}

最后是ServiceProviderController：

 public ActionResult Create()
    {
        var _allServiceTypes =  Enum.GetValues(typeof(ServiceTypeEnum))
           .Cast<ServiceTypeEnum>()
           .Select(t => new KeyValuePair<string, int>(t.ToString(), (int) t));

        var viewModel = new ServiceProviderViewModel.CreateModel()
        {
            AllServiceTypes = _allServiceTypes
        };

        return View(viewModel);
    }

    [HttpPost]
    public ActionResult Create(ServiceProviderViewModel.CreateModel viewModel)
    {
        if (ModelState.IsValid)
        {
            // This is OK for a single Service Type
            //var serviceProvider = new ServiceProvider
            //{
            //    Title = viewModel.Title,
            //    ServiceType = viewModel.ServiceType
            //};

            // For multiple Service Types
            var serviceProvider = new ServiceProvider();
            serviceProvider.Title = viewModel.Title;
            for (int i = 0; i < viewModel.SelectedServiceTypes.Length; i++)
            {
                serviceProvider.ServiceTypes.Add((ServiceTypeEnum)Enum.Parse(typeof(ServiceTypeEnum), viewModel.SelectedServiceTypes[i].ToString()));
            }

            repository.InsertServiceProvider(serviceProvider);


            repository.Save();
            return RedirectToAction("Index");
        }
        return View(viewModel);
    }

问题是；数据库中没有用于添加的服务类型的表或列。在调试时，我可以看到复选框的选定值被转换为相应的枚举并添加到 serviceProvider.ServiceTypes

当我尝试获取服务类型时什么都没有：

repository.ServiceProviders.ServiceTypes

我错过了什么？

Answer 1

枚举是一个结构。只有类类型可以被持久化。实体框架正确地忽略了“关系”。你如何想象一个枚举首先会被表示为一个数据库表？