ASP.NET MVC 4, EF5, Unique property in model - best practice?

Question

ASP.NET MVC 4, EF5, Code First, SQL Server 2012 Express

What is best practice to enforce a unique value in a model? I have a places class that has a

Answer 1

Faced similar issue in my ASP.NET Razor Page Project. Creating custom UniqueDataAttribute didn't work, because on Edit, it would throw an error if you're not changing unique field.

I needed unique Book Name. This is how I resolved:

1. I added unique constraint to the field in database via EF Core migrations. Added following in ApplicationDbContext class and then ran migration.

Code:

protected override void OnModelCreating(ModelBuilder builder)
        {
            builder.Entity()
                .HasIndex(u => u.Name)
                .IsUnique();
        }

2. Next, created helper/extension method as follows.

Code:

        // Validate uniqueness of Name field in database.
        // If validation is done on existing record, pass the id of the record.
        // Else, if validating new record Name, then id is set to dummy key integer -1
        public static bool UniqueNameInDb(this string data, ApplicationDbContext db, int id = -1)
        {
            var duplicateData = from o in db.Book
                                where o.Name == data && o.Id != id
                                select o;
            if(duplicateData.Any())
            {
                return false;
            }
            return true;
        }
    }

3. Then used it in Create and Edit page model in OnPost() method as follows.

Create model:

public async Task OnPost()
        {
            if(ModelState.IsValid)
            {
                if (!Book.Name.UniqueNameInDb(_db)) //<--Uniqueness validation
                {
                    ModelState.AddModelError("Book.Name", "Name already exist"); //<-- Add error to the ModelState, that would be displayed in view.
                    return Page();
                }

                await _db.Book.AddAsync(Book);
                await _db.SaveChangesAsync();

                return RedirectToPage("Index");

            }
            else
            {
                return Page();
            }
        }

Edit Model:

public async Task OnPost()
        {
            if(ModelState.IsValid)
            {
                var bookFromDb = await _db.Book.FindAsync(Book.Id);
                if (!Book.Name.UniqueNameInDb(_db, Book.Id)) //<--Uniqueness validation
                {
                    ModelState.AddModelError("Book.Name", "Name already exist"); //<-- Add error to the ModelState, that would be displayed in view.
                    return Page();
                }
                bookFromDb.Name = Book.Name;
                bookFromDb.Author = Book.Author;

                await _db.SaveChangesAsync();

                return RedirectToPage("Index");
            }

            return Page();
        }

PS: Your Razor view should've Model validation set on in the form to capture and display the error.

i.e,

and below validation against the field.