實體框架實體中延遲加載時出錯

Question

我的網頁上有錯誤：

{"The ObjectContext instance has been disposed and can no longer be used for operations that require a connection."}

它在這里出現了我的看法：

@model Service_Monitor_Web_Interface.Models.DashBoardViewModel
...    
=> @Html.DisplayFor(modelItem => item.Service.Name)

這是我的ViewModel類：

    public class DashBoardViewModel
{

    public int NumberOfUsers { get; set; }

    public virtual IEnumerable<ApplicationUser> recentUsers { get; set; }

    public virtual IEnumerable<UserActivityLog> logs { get; set; }

    public virtual IList<Service_Monitor_Domain.Entities.ServiceStatusHistory> serviceStatus { get; set; }

}

這是我班級的控制器：

   public ActionResult Index()
    {
        DashBoardViewModel vm = new DashBoardViewModel();  

        var dbs = new EFServiceStatusHistoryRepository();

        vm.serviceStatus = dbs.GetAllEnabledLatestStatusLog();

        return View(vm);
    }

這是我訪問db的函數：

  public List<ServiceStatusHistory> GetAllEnabledLatestStatusLog()
    {
        List<ServiceStatusHistory> list = null;
        try
        {
            using (var db = new EFDbContext())
            {
                //var results = db.ServiceStatusHistory.Where(h => h.Service.Enabled)
                //                  .OrderByDescending(h => h.Id)
                //                  .GroupBy(h => h.Service.Id)
                //                  .Select(grp => grp.FirstOrDefault());

                var results = db.ServiceStatusHistory
                            .Where(x => x.Service.Enabled)
                            .GroupBy(x => x.Service)
                             .Select(x => x.OrderByDescending(y => y.time).FirstOrDefault());

                list = results.ToList();
            }
            return list;
        }
        catch
        {
            //Error
        }
        return list;
    }

這是我的實體：

    public class ServiceStatusHistory
{
    [Key]
    [Required]
    public int Id { get; set; }

    // Forenkey to service
    [Required]
    public virtual Service Service { get; set; }

    [Required]
    public ServiceStatus Status { get; set; }

    [Required]
    public string Messages { get; set; }

    [Required]
    //Time Logged in the db
    public DateTime time { get; set; }

    [Required]
    //Time the service last called the update method on the client
    public DateTime LastUpdateTime { get; set; }
 }

我認為它與延遲加載有關。 但是在查詢同一個表之前我沒有遇到過這個問題？ 然而，這只是一個簡單的選擇查詢。

Answer 1

考慮在GetAllEnabledLatestStatusLog（）中加載Service類的“Eager”。 請務必使用System.Data.Entity
using System.Data.Entity var results = db.ServiceStatusHistory .Include("Service") .Where(x => x.Service.Enabled) .GroupBy(x => x.Service) .Select(x => x.OrderByDescending(y => y.time).FirstOrDefault()); 請參閱MSDN加載相關實體

Answer 2

我說這里的問題是在你的查詢中你沒有顯式加載相關的實體（在這種情況下， ServiceStatusHistory.Service ），大概是Lazy Loading默認開啟。

所以稍后當你引用item.Service.Name ，底層的EF類意識到它需要加載這些相關的實體 - 除了關聯的ObjectContext （在這種情況下， EFDbContext ）早已被處理掉。

因此，解決此問題的最簡單方法可能是將查詢修改為.Include（）相關的Service實體

 var results = db.ServiceStatusHistory
                    .Include("Service")
                    .Where(x => x.Service.Enabled)
                    .GroupBy(x => x.Service)

                    .Select(x => x.OrderByDescending(y => y.time).FirstOrDefault());

或者您可以關閉該查詢的延遲加載 ：

using (var db = new EFDbContext())
{
   db.Configuration.LazyLoadingEnabled=false;
   //use original query here
}