ASP.NET MVC 2: Model Validation - username already taken? - validation

Question

I am following Scott Gu's blog: here
In his Blog he talks about client and server side validation.
How does one validate if username has already been taken and display this as a validation error message to the user?
In Scott's blog, this would be the same as validating if Title is unique:
public class Dinner
{
public int DinnerID { get; set; }
[Required(ErrorMessage = "Please enter a Dinner Title")]
[StringLength(20, ErrorMessage = "Title is too long")]
public string Title { get; set; }
[Required(ErrorMessage = "Please enter the Date of the Dinner")]
public DateTime EventDate { get; set; }
[Required(ErrorMessage = "Please enter the location of the Dinner")]
[StringLength(30, ErrorMessage = "Address is too long")]
public string Address { get; set; }
[Required(ErrorMessage = "Please enter your email address")]
[RegularExpression(".+\\#.+\\..+", ErrorMessage = "Please enter a valid email address")]
public string HostedBy { get; set; }
public virtual ICollection<RSVP> RSVPs { get; set; }
}
My first guess is that somehow this is done within the Model Controller, here:
//
// POST: /Home/Create
[HttpPost]
public ActionResult Create(Dinner dinner)
{
if (ModelState.IsValid)
{
nerdDinners.Dinners.Add(dinner);
nerdDinners.SaveChanges();
return RedirectToAction("Index");
}
return View(dinner);
}
And because the Title is stored in a database server, this would be server side validation.
I know how to check if the Title is unique, but I do not know how to make the validation message appear in the View like it does using declaratives like [Required] or [StringLength()]. For example, here is how I can check for uniqueness:
[HttpPost]
public ActionResult Create(Dinner dinner)
{
if (ModelState.IsValid)
{
foreach (var existingDinner in nerdDinners.Dinners)
{
if(existingDinner.Title == dinner.Title)
{
**// TODO: display validation error message?**
}
}
nerdDinners.Dinners.Add(dinner);
nerdDinners.SaveChanges();
return RedirectToAction("Index");
}
return View(dinner);
}
Using my imagination, and a magic wand, I would want to create a new declarative called [TitleIsUnique] that performs like the other validation declaratives.
Thank you in advance for your assistance.

Answer 1

You could create a custom attribute as mentioned and use IValidateObject but I prefer to add my errors to the ModelState in one of the layers in my application.
For this you can use ModelState.AddModelError
If you use ModelState.AddModelError("Title", "Title must be unique"); it will add an error to the Title field.
If you use ModelState.AddModelError("*", "Title must be unique"); it will add a general error message for the page.
[HttpPost]
public ActionResult Create(Dinner dinner)
{
if (ModelState.IsValid)
{
if(nerdDinners.Dinners.Any(d => d.Title == dinner.Title))
{
ModelState.AddModelError("Title", "The title is not unique");
return View(dinner);
}
nerdDinners.Dinners.Add(dinner);
nerdDinners.SaveChanges();
return RedirectToAction("Index");
}
return View(dinner);
}

Answer 2

You are probably looking at implementing your own attribute derived from CustomAttribute. Take a look at this blog post http://blogs.msdn.com/b/adonet/archive/2011/05/27/ef-4-1-validation.aspx - it shows how to validate uniqueness. In the post IValidatableObject interface is used to perform validation but you should be able to the same by creating CustomAttribute.