无法将以前插入的对象添加到另一个对象（实体框架）

Question

我有EF课程OAFile和电子邮件。我正在从服务器加载电子邮件并存储在存储库中。然后，用户可以列出电子邮件和＆＃34;关联＆＃34;他们使用所选文件。

电子邮件对象先前已插入存储库，我想获取该文件的电子邮件数。

file.Emails.Add(email) 

将电子邮件添加到文件对象，但不会保存。

file.Emails.Count 

总是返回0。

正确保存email.FileID字段。

我错过了什么？如何通过调用file.Emails.Count来获取电子邮件的数量？

    [HttpPost]
    [ValidateInput(false)]
    public ActionResult AssignEmail(int emailId, int assignedTo)
    {
        if (emailId > 0 && assignedTo > 0)
        {
            Email email = repository.Emails.FirstOrDefault(x => x.EmailID == emailId);
            OAFile file = repository.OAFiles.FirstOrDefault(x => x.FileID == assignedTo);
            if (email != null && file != null)
            {
                email.FileID = assignedTo;
                email.AssignedByID = HttpContext.User.Identity.GetUserId();
                file.Emails.Add(email);
                repository.Save();
                return Json(new { success = true }, JsonRequestBehavior.DenyGet);
            }
            else
            {
                return Json(new { success = false }, JsonRequestBehavior.DenyGet);
            }
        }
        else
        {
            return Json(new { success = false }, JsonRequestBehavior.DenyGet);
        }
    }

电子邮件类：

public class Email
{
    public Email()
    {
        Attachments = new HashSet<EmailAttachment>();
    }

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

    public string EmailIdentifier { get; set; }

    public int MessageNumber { get; set; }

    public string From { get; set; }

    public string Subject { get; set; }

    public DateTime DateSent { get; set; }

    public string Body { get; set; }

    public int? FileID { get; set; }

    public string AssignedByID { get; set; }

    public string AssignedToID { get; set; }

    public string EmailType { get; set; }

    public ICollection<EmailAttachment> Attachments { get; set; }

    [ForeignKey("FileID")]
    public virtual OAFile OAFile { get; set; }

    [ForeignKey("AssignedByID")]
    public virtual AppUser AssignedBy { get; set; }

    [ForeignKey("AssignedToID")]
    public virtual AppUser AssignedTo { get; set; }
}

和OAFile类：

public class OAFile
{
    public OAFile()
    {
        Services = new HashSet<Service>();
        Documents = new HashSet<Document>();
        Notes = new HashSet<Note>();
        Forms = new HashSet<Form>();
        Emails = new HashSet<Email>();
    }

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

    [Required]
    public int CompanyID { get; set; }

    [Required]
    [StringLength(14)]
    public string OurFileName { get; set; }

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

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

    [DisplayFormat(DataFormatString = "{0:dd.MM.yyyy}")]
    public DateTime CreatedOn { get; set; }

    public int ClientID { get; set; }


    [ForeignKey("ClientID")]
    public virtual Client Client { get; set; }

    public virtual ICollection<Service> Services { get; set; }

    public ICollection<Document> Documents { get; set; }

    public ICollection<Note> Notes { get; set; }

    public ICollection<Form> Forms { get; set; }

    public ICollection<Email> Emails { get; set; }

    public virtual Company Companies { get; set; }

    [ForeignKey("AppUserId_Creator")]
    public virtual AppUser AppUsers_Creator { get; set; }
}

编辑：我正在调用下面的file.Emails.Count语句来检查文件是否有与之关联的电子邮件。

 public bool IsFileEmpty(int fileId)
    {
        bool isFileEmpty = true;

        OAFile file = repository.FindOAFile(fileId);
        if (file.Services.Count > 0 || file.Documents.Count > 0
            || file.Forms.Count > 0 || file.Notes.Count > 0 || file.CCTable != null || file.AccountingTable != null
            || file.Emails.Count > 0
            )
        {              
            isFileEmpty = false;
        }
        return isFileEmpty;
    }

编辑2：我正在为另一个控制器（HomeController.cs）调用IsFileEmpty（）方法，同时为搜索结果填充视图模型。如果文件为空，结果行将显示“删除”链接。

public ActionResult FilesSearch(FileSearchViewModel viewModel)
    {
        var files = !string.IsNullOrWhiteSpace(viewModel.Keyword) ? repository.FindOAFiles(viewModel.Keyword) : repository.OAFiles;
        var sortedFile = files.Where(x => x.IsFileOpen).OrderByDescending(x => x.CreatedOn).ToList();
        sortedFile.AddRange(files.Where(x => !x.IsFileOpen).OrderByDescending(x => x.CreatedOn));

        var resultsList = new List<FileSearchResultsViewModel>();

        foreach (OAFile file in sortedFile)
        {
            var resultsViewModel = new FileSearchResultsViewModel();
            resultsViewModel.oAFile = file;
            resultsViewModel.isFileEmpty = IsFileEmpty(file.FileID);

            resultsList.Add(resultsViewModel);
        }
        return PartialView("_FilesSearch", resultsList);
    }