How to validate key constraints in ASP.NET MVC

Question

I use the built in model validation which works great for specifying required fields and such but on my model I have specified key constraints so the combination of two columns are unique.

How should I validate for this so it doesn't throw exception if user tries to add duplicate?

Here is my model:

public class EmailFilter
{
    public int ID { get; set; }
    [Required]
    [StringLength(100)]
    [Index("IX_FilterAndEmail", 2, IsUnique = true)]
    public string Email { get; set; }
    //This is an enum 
    [Required]
    [Index("IX_FilterAndEmail", 1, IsUnique = true)]
    public EmailFilterType Filter { get; set; }
}

And my create controller method. I tried to add an error but I am not doing it right. It stops the exception happening but it just returns to the listview. I'm not sure what the right way to validate here is.

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create([Bind(Include = "ID,Email,Filter")] EmailFilter emailFilter)
    {
        if (ModelState.IsValid)
        {
            if(db.EmailFilters.Any(x => x.Filter == emailFilter.Filter && x.Email == emailFilter.Email))
            {
                // This doesn't seem to do anything and returns to listview not create view
                // "EFValidationSummary" is the id of my validation summary razor control
                ModelState.AddModelError("EFValidationSummary", "This filter already exists");
                return View(emailFilter);
            }
            else
            {
                db.EmailFilters.Add(emailFilter);
                db.SaveChanges();
            }

            return RedirectToAction("Index");
        }

        return View(emailFilter);
    }

How do I properly trigger an error and send back to the create page with the validation error displayed?

Answer 1

The first parameter of AddModelError() is the name of the property in your model. In your case, the error message would be displayed in the placeholder generated by

@Html.ValidationMessageFor(m => EFValidationSummary)

but your model does not contain a property named EFValidationSummary . In order to display validation messages in the placeholder generated by @Html.ValidationSummary() , you need to provide an empty string for the first parameter

 ModelState.AddModelError("", "This filter already exists");