如何在Entity Framework中获取新条目的主键并设置FK关系

Question

In my ASP.NET MVC 5 application, I have situation where I need to get the primary key that got generated for a particular entry in database. Also, I need to get that id and set a foreign key relationship in database. I have tried below attempt and it seems to be working but I had to call _context.SaveChanges() TWICE.

UI Behaviour: the user has a dropdown with a list of companies in referral form. When a company is not found, user will select "Others" in the dropdown. And it will show a textbox, where user will enter the company name. So I need to add this company to the database and link it to the referral row.

Relationship between tables Referral and Company :

 dBModelBuilder.Entity<Company>()
            .HasMany(u => u.Referrals)
            .WithRequired(u => u.Company)
            .HasForeignKey(u => u.CompanyId);

Model classes:

public class Company
{
    [Key]
    public int CompanyId { get; set; }
    public string CompanyName { get; set; }

    public ICollection<Referral> Referrals { get; set; }
    public ICollection<CoverLetter> CoverLetters { get; set; }
}

public class Referral
{
    [Key]
    public int ReferralId { get; set; }

    [Display(Name = "REFERRALS")]
    public string ReferralName { get; set; }

    public int? CoverLetterId { get; set; }
    public virtual CoverLetter CoverLetter { get; set; }

    public int CompanyId { get; set; }
    public virtual Company Company { get; set; }
}

ViewModel:

public class Referral
{
    [Key]
    public int ReferralId { get; set; }

    [Display(Name = "REFERRALS")]
    public string ReferralName { get; set; }

    public int? CoverLetterId { get; set; }
    public virtual CoverLetter CoverLetter { get; set; }

    public int CompanyId { get; set; }
    public virtual Company Company { get; set; }
}

My attempt inside the controller action:

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create(ReferralViewModel viewModel)
    {
        var candidateId = User.Identity.GetUserId();

        var referral = new Referral
        {
            ReferralName = viewModel.ReferralName,
        };

        if (viewModel.CompanyId.HasValue)
        // if it is option 4 then we need to add it to the company table
        {
            if (!string.IsNullOrEmpty(viewModel.TempCompany))
            {
                var f = new Company
                {
                    CompanyName = viewModel.TempCompany
                };

                _context.Companies.Add(f);
               // ********FIRST CALL
                 _context.SaveChanges(); 

                _context.Referrals.Add(referral);
                 referral.CompanyId = f.CompanyId;
               // **********SECOND CALL
                _context.SaveChanges(); // SECOND CALL ------
            }
            else
            {
                referral.CompanyId = viewModel.CompanyId.Value;
                _context.Referrals.Add(referral);
                _context.SaveChanges();
            }
        }

        return RedirectToAction("ReferralCenter");
    }

Question: can I do these steps in one call to _context.SaveChanges(); ?

EDIT

With this code, I get a NullReferenceException :

if (!string.IsNullOrEmpty(viewModel.TempCompany))
{
    var f = new Company
                {
                    CompanyName = viewModel.TempCompany
                };

    f.Referrals.Add(referral); // Referrals is NULL(EXCEPTION)
    _context.Companies.Add(f);
    // _context.SaveChanges();
    //_context.Referrals.Add(referral);
    // referral.CompanyId = f.CompanyId;
    _context.SaveChanges();
}

Answer 1

If you have the navigation property defined, you could just save both the Company and Referral in one shot as follows:

if (!string.IsNullOrEmpty(viewModel.TempCompany))
{
   var f = new Company
   {
        CompanyName = viewModel.TempCompany
   };
   referral.Company = f;
   _context.SaveChanges();
}

EF will take care of setting the FK.

EDIT

Have updated the code to include _context.Referrals.Add(referral) as mentioned in the comments:

if (!string.IsNullOrEmpty(viewModel.TempCompany))
{
   var f = new Company
   {
        CompanyName = viewModel.TempCompany
   };

   referral.Company = f;
   _context.Referrals.Add(referral);
   _context.SaveChanges();
}

Answer 2

It is my understanding that FK constraints are managed within EF, so long as they are configured in the DB.

If in your EF model the Referral is a child of Company then there should be a referrals collection accessible from the Company object instance.

UPDATE I just built a simple sample MVC Proj, 2x SQL tables (Referrals and Company FK on CompanyId) 4 x Companies, 4th being OTHER, built an EF model with Reverse POCO for speed, added a Controller with scaffolding and views and modified my controller to the below. (I just insert the date time rather than tempCompanyName. The syntax generated here is more like that in the answer by @ashin, but it works for me.

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create([Bind(Include = "ReferralId,CompanyId")] Referral referral)
    {
        if (ModelState.IsValid)
        {
            if (referral.CompanyId == 4)
            {
                var f = new Company() { Name = DateTime.Now.ToString() };
                referral.Company = f;
            }
            db.Referrals.Add(referral);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        ViewBag.CompanyId = new SelectList(db.Companies, "CompanyId", "Name", referral.CompanyId);
        return View(referral);
    }

My Company model does instanciate a new Referrals Collection

    public Company()
    {
        Referrals = new System.Collections.Generic.List<Referral>();
    }