How do i post information from my Registration Form to the AspNetUser table in MVC 5

Question

I am working on an application where students get to select their department, and role while registering to the site. I was able to add the option to select Department and role to the registration form. Actually, the DropDownList for the Department is being Populated from a Department Table in the database. but When I try to register a user after filling out the form, I get this error message

Server Error in '/' Application. The INSERT statement conflicted with the FOREIGN KEY constraint "FK_dbo.AspNetUsers_dbo.Departments_DepartmentID". The conflict occurred in database "aspnet-AptechAPP-20180514123201", table "dbo.Departments", column 'DepartmentID'. The statement has been terminated.

Below is the Screen shot of my RegisterViewModel

public class RegisterViewModel
{
    [Required]
    [EmailAddress]
    [Display(Name = "Email")]
    public string Email { get; set; }

    [Required]
    [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
    [DataType(DataType.Password)]
    [Display(Name = "Password")]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm password")]
    [Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
    public string ConfirmPassword { get; set; }
    public int DepartmentID { get; set; }
    public IEnumerable<System.Web.Mvc.SelectListItem> DepartmentList
    {
        get; set;
    }
}

My RegisterViewController

 // GET: /Account/Register
    [AllowAnonymous]
    public ActionResult Register()
    {
        ViewBag.Name = new SelectList(context.Roles.Where(u => !u.Name.Contains("Admin"))
            .ToList(), "Name", "Name");
        RegisterViewModel model = new RegisterViewModel();
        ConfigureRegisterViewModel(model);
        return View(model);
    }

    //
    // POST: /Account/Register
    [HttpPost]
    [AllowAnonymous]
    [ValidateAntiForgeryToken]
    public async Task<ActionResult> Register(RegisterViewModel model)
    {
        if (!ModelState.IsValid)
        {
            ConfigureRegisterViewModel(model);
            return View(model);
        }

        var user = new ApplicationUser { UserName = model.Email, Email = model.Email, DepartmentID = model.DepartmentID };


        var result = await UserManager.CreateAsync(user, model.Password);
        if (result.Succeeded)
        {
            await SignInManager.SignInAsync(user, isPersistent: false, rememberBrowser: false);

            // For more information on how to enable account confirmation and password reset please visit http://go.microsoft.com/fwlink/?LinkID=320771
            // Send an email with this link
            // string code = await UserManager.GenerateEmailConfirmationTokenAsync(user.Id);
            // var callbackUrl = Url.Action("ConfirmEmail", "Account", new { userId = user.Id, code = code }, protocol: Request.Url.Scheme);
            // await UserManager.SendEmailAsync(user.Id, "Confirm your account", "Please confirm your account by clicking <a href=\"" + callbackUrl + "\">here</a>");

            return RedirectToAction("Index", "Home");
        }
        ViewBag.Name = new SelectList(context.Roles.Where(u => !u.Name.Contains("Admin"))
            .ToList(), "Name", "Name");

        AddErrors(result);


        // If we got this far, something failed, redisplay form
        ConfigureRegisterViewModel(model);
        return View(model);
    }
    private void ConfigureRegisterViewModel(RegisterViewModel model)
    {
        IEnumerable<Department> departments = db.Departments.OrderBy(u => u.DepartmentName).ToList();
        model.DepartmentList = departments.Select(a => new SelectListItem
        {
            Value = a.DepartmentID.ToString(),
            Text = a.DepartmentName.ToString()
        });


    }

My DepartmentViewModel

 public class Department
{
    public virtual int DepartmentID { get; set; }
    public virtual string DepartmentName { get; set; }
}

My IdentityModel

public class ApplicationUser : IdentityUser
{
    public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<ApplicationUser> manager)
    {
        // Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
        var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
        // Add custom user claims here
        return userIdentity;
    }
     public int DepartmentID { get; set; }
    public virtual Department Department
    {
        get; set;
    }
}

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    public ApplicationDbContext()
        : base("DefaultConnection", throwIfV1Schema: false)
    {
    }

    public static ApplicationDbContext Create()
    {
        return new ApplicationDbContext();
    }
}

Register.Cshtml

 <div class="form-group">
    @Html.LabelFor(model => model.DepartmentID, new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        @Html.DropDownListFor(m => m.DepartmentID, Model.DepartmentList, "Please select", new { @class = "form-control" })

    </div>
</div>
<div class="form-group">
    @Html.Label("user Role", new { @class = "col-md-2 control-label" })
    <div class="col-md-10">
        @*@Html.DropDownList("Name")*@
        @Html.DropDownList("UserRoles", (SelectList)ViewBag.Name, " ")
    </div>
</div>

Answer 1

The target table you are inserting data into has a foreign key relationship with Department ID in another table. Before data can be inserted, the Department ID must exist in the other table first. Is a valid department ID selected? What is the Department ID when selected? Does that department ID exist in the other table?

var user = new ApplicationUser 
{ UserName = model.Email, 
Email = model.Email, 
DepartmentID = model.DepartmentID //here?
};

Answer 2

I think that the problem is that you are not building a correct Dropdownlist in View, so when form is submitted, the application is triying to insert null value in DepartmentId of the user and it throws an exception because is an invalid FK.

In view, change this:

@Html.DropDownListFor(
m => m.DepartmentID, 
Model.DepartmentList, 
"Please select", 
new { @class = "form-control" })

To this:

@Html.DropDownListFor(
    m => m.DepartmentID,
    new SelectList(Model.DepartmentList, "Value", "Text"), 
    "Please select", 
    new { @class = "form-control" })