title: Custom validation based on other value
body: <p>I make a Booking form for restaurant, which asks for the name of the restaurant, the date of the meal and the number of person.</p>
  
  <p>I have a booking class, which has an ID, an ID of the restaurant, a date and a number of people :</p>
  
  <pre><code>public class Booking
  {
      public int Id { get; set; }
      public int IDRestaurant{ get; set; }
      [CustomPlaceValidator]
      public int Nbpeople { get; set; }
      [CustomDateValidator]
      public DateTime Date { get; set; }
  }
  </code></pre>
  
  <p>As well as a Resto class, which has an ID, a name, phone number and a number of table :</p>
  
  <pre><code>public class Resto
  {
      public int Id { get; set; }
      [Required(ErrorMessage = "Le nom du restaurant doit être saisi")]
      public string Nom { get; set; }
      [Display(Name = "Téléphone")]
      [RegularExpression(@"^0[0-9]{9}$", ErrorMessage = "Le numéro de téléphone est incorrect")]
      public string Telephone { get; set; }
      [Range(0, 9999)]
      public int Size { get; set; }
  }
  </code></pre>
  
  <p>I would like to make a validation to check with each new reservation, that the restaurant is not full.
  To do this, when validating the "Number of persons" field of the Booking, I need the value of the "restaurant name" field and the value of the "date" field, and then retrieve all the bookings on this Restaurant at that date, and check whether the sum of the number of persons is much lower than the capacity of the restaurant.</p>
  
  <pre><code>public class CustomPlaceValidator : ValidationAttribute
  {
      private IDal dal = new Dal();
      protected override ValidationResult IsValid(object value, ValidationContext validationContext)
      {
          int nb = 0;
          if (dal.GetAllBooking() != null)
          {
              foreach (var booking in dal.GetAllBooking())
                  nb += booking.Nbpeople;
              if (nb ..... ) return ValidationResult.Success;
              return new ValidationResult("The restaurant is full for this date.");
          }
          return ValidationResult.Success;
  
      }
  
  }
  </code></pre>
  
  <p>(It's a draft, the tests are not finished obviously)</p>
  
  <p>How can I have the value of the other proprieties for my validation ?</p>