Cannot Add Previously Inserted Object to Another Object (Entity Framework) - asp.net-mvc

Question

I have the EF classes OAFile and Email. I am loading the emails from the server and storing in the repository. The user can then list the e-mails and "associate" them with the selected file.
The email object is previously inserted into the repository, and I want to get the count of the emails of the file.
file.Emails.Add(email)
adds the email to the file object, but it is not being saved.
file.Emails.Count
returns always 0.
The email.FileID field is saved correctly.
What am I missing? How can get the number of email by calling file.Emails.Count?
[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);
}
}
The Email class:
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; }
}
and the OAFile class:
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; }
}
EDIT: I am calling the file.Emails.Count statement below to check whether the file has any emails associated with it.
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;
}
EDIT 2: I am calling the IsFileEmpty() method in another controller (HomeController.cs) while populating view model for search results. The results rows display the Delete link if the file is empty.
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);
}