How To Use Partial Method To Validate Fields Aganist Data In Database

i want to make sure all product names are unique so i tried to do the following. but it is causing an infinity loop at the lambda expression.

public partial class Product
{
partial void OnProductNameChanging(string value)
{
using(var ctx = new MyEntityContext()){
var val = value;
var item = ctx.Products.Where(o=>o.ProductName == val).FirstOrDefault();
if(item != null)
throw new ValidationException("Product Name existed.");
}
}
}

i'm using asp.net 4.0 with dynamic data and entity framework.