asp.Net Core one-to-many relationship UPDATE statement conflicted with the FOREIGN KEY constraint

Question

I am using Entity Framework Core and created one-to-many relationship between 2 tables:

public class Fgf
  {
     public int FgfID { get; set; }

    public string FGFName { get; set; }
    ...
    public ICollection<ApplicationUser> ApplicationUsers { get; set; }

}


 public class ApplicationUser : IdentityUser
    {
    public string FirstName { get; set; }
    public string LastName { get; set; }
...
}

In this case EFCore is adding automatically FgfID column in ApplicationUser, but I cannot access it in the controller.

I am trying to add the FgfID in the ApplicationUser

 public class ApplicationUser : IdentityUser
    {
    public string FirstName { get; set; }

    public string LastName { get; set; }
...

   public int FgfID { get; set; }
   public Fgf Fgf { get; set; }
}

I have a form, where I create an FGF and assign to him a user with a selectlist. I am using a simple form with POST action. I am not using model for the form as I have another form on this Razor Page.

In the controller I have:

  [HttpPost]
  public async Task<IActionResult> NewFgf(string FGFName, string ApplicationUser)
    {
        if (ModelState.IsValid) {
            var fgf = new Fgf
            {
                FGFName = FGFName,                    
            };

            var usertoupdate = await _context.Users
                .Where(c=> c.Id == ApplicationUser)
                .SingleOrDefaultAsync();

            usertoupdate.FgfID = fgf.FgfID;
            
            _context.Users.Update(usertoupdate);
            _context.Add(fgf);             
            await _context.SaveChangesAsync();
            return RedirectToAction(nameof(Index));              
        }

        return View();
    }

The error I get is:

SqlException: The UPDATE statement conflicted with the FOREIGN KEY constraint "FK_AspNetUsers_Fgf_FgfID". The conflict occurred in database "aspnet-StreamPixelVer2-3E2C2112-15B8-48A4-8619-4773C5ADBFBD", table "dbo.Fgf", column 'FgfID'. The statement has been terminated.

What am I doing wrong? What is the best practice to save the data to the DB with Many-To-One, One-To-Many relationship with the Asp.net Core?

Answer 1

Since you don't have fgf.FgfID, try this:

using Microsoft.EntityFrameworkCore.ChangeTracking;

.....
....

var usertoupdate = await _context.Set<ApplicationUser>()
                .Where(c=> c.Id == applicationUser.Id)
                .SingleOrDefaultAsync();

      var fgf = new Fgf
            {
                FGFName = FGFName,                    
            };  
        

  fgf.ApplicationUsers = new ObservableCollectionListSource<ApplicationUser>();
  fgf.ApplicationUsers.Add(usertoupdate);  
    _context.Set<Fgf>().Add(fgf);
   await _context.SaveChangesAsync();