Unique constraint with EFCodeFirst and SqlCe4

Question

I'm writing an ASP.net MVC3 application using Entity Framework Code First and SqlCe4.

I have been designing several models, and found one that is turning out to be interesting. This is what I have so far:

public class Preference
    {
        public int PreferenceId { get; set; }

        [Required]
        public int StudentId { get; set; }
        public virtual Student Student { get; set; }

        [Required]
        public int PresentationId { get; set; }
        public virtual Presentation Presentation { get; set; }

        [Required]
        public int Rank { get; set; }
    }

I however, need a unique constraint or index, because for a particular student, I want them to have to have a list of preferences, but the PresentationId needs to be unique for each student. Is there a way to do this via Code First or some validation attribute?

This sounds like I'm going down the branch of a many to many relationship with the Preference object being an intermediary, to add the Rank property. However, I can't seem to find how to make sure that the values are unique. Is the best way really to manually just add a unique index to the database outside of EF?

Answer 1

Currently(ie EF 4.1) EF does not have an Attribute or configuration mechanism to create unique indexes.

However if you are using Database Initializer you can create it manually

public class MyInitializer : IDatabaseInitializer<MyContext>
{
    public void InitializeDatabase(MyContext context)
    {
        if (!context.Database.Exists() || !context.Database.ModelMatchesDatabase())
        {
            context.Database.DeleteIfExists();
            context.Database.Create();

            context.ObjectContext.ExecuteStoreCommand("CREATE INDEX IX_Preference_PresentationId ON Preference ( PresentationId )");
        }
    }
}

Or execute it outside the Initializer.