public interface IWithType
    {
        /// <summary>
        /// Тип элемента
        /// </summary>
        [Association(OnTargetRemove = OnRemoveAction.Cascade)]
        [Field(Nullable = false)]
        DocEntity ItemType { get; set; }
    }

public abstract class FinToolBase : Entity, IWithType
    {
/// <summary>
        /// Тип элемента
        /// </summary>
        [Field(NullableOnUpgrade = true)]
        public DocEntity ItemType { get; set; }
}

This setup fails with:

Error   3   An unexpected exception occured when executing user code: System.InvalidOperationException: Sequence contains no matching element
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
   at Xtensive.Storage.PersistentAspect.ProvideAspects(Object targetElement) in d:\Home\dataobjects.net\Xtensive.Storage\Xtensive.Storage\Aspects\PersistentAspect.cs:line 56
   at PostSharp.Sdk.Utilities.ExceptionHelper.ExecuteUserCode[T](Func`1 userCode, Type[] acceptableExceptions)  D:\Home\Docs\Visual Studio 2010\Projects\Eis\Eis.Models\POSTSHARP   Eis.Models