Using a ASP .NET MVC model value to validate another model value?

Question

I have a model class like this:

public class SomeClass
{
    public int EmployeeId {get;set;}
    public int DayTotal {get;set}
}

For this model class I am creating a custom ValidationAttribute for the 'DayTotal' property. The custom validator will check the entered value 'DayTotal' value against another table that defines the maximum days allowed.

How to I refer to the selected 'EmployeeId' from the Create view when writing my query in the validator?

public class DayTotalAttribute : ValidationAttribute
{
    ProjectDBContext db = new ProjectDBContext();

    public override bool IsValid(object value)
    {
        if (value == null)
        {
            return false;
        }

        var products = from p in db.EmployeeDayMax
                       where p.EmployeeId = ???
    }
}

UPDATE:

My solution has taken a different approach. The helpful answers got me looking in other places. This blog from Scott Gu helped to provide a simpler approach:

In my model ->

   public class SomeClass : IValidateObject
   {
         public int EmployeeId {get; set;}
         public int DayTotal {get; set;}

         public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
         {
            ProjectDBContext db = new ProjectDBContext();
            //check maxes...can refer directly to EmployeeId in LINQ queries
            if(failed)
            {
               yield return new ValidationResult("Days have been maxed!");
            }

Answer 1

@Luke Bennett is completely right, ValidationContext is new in MVC 3.

Here's an example using your code:

public class DayTotalAttribute : ValidationAttribute
{
    ProjectDBContext db = new ProjectDBContext();

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value != null)
        {
            var model = (SomeClass)validationContext.ObjectInstance;
            var products = from p in db.EmployeeDayMax
                       where p.EmployeeId = model.EmployeeId

            bool somethingIsWrong = // do your validation here

            if (somethingIsWrong)
            {
                return ValidationResult("Error Message");
            }
        }

        return base.IsValid(value, validationContext);
    }    
}

Answer 2

In .NET 4.0 there is another overload for IsValid that takes a ValidationContext parameter. This has an ObjectInstance property which you can cast to SomeClass and gives you access to its other properties.