[HierarchyRoot]
public class Login2Partner : Entity
{
    public Login2Partner(Session session) : base(session)
    {
    }

    [Key]
    [Field]
    public Guid ID { get; private set; }

    [Field(Indexed = true)]
    [PastConstraint]
    public DateTime Inserted { get; set; }

    [Field(Nullable = false)]
    public Login Login { get; set; }

    [Field(Indexed = true, Nullable = false)]
    [NotNullOrEmptyConstraint]
    public string Partner { get; set; }

    [Field(Indexed = true, Nullable = false)]
    [NotNullOrEmptyConstraint]
    public string Key { get; set; }

    [Field(Nullable = true, Indexed = true)]
    public string Campaign { get; set; }
}

throw exception on validate:

System.InvalidOperationException: Can not commit a transaction. Entities validation failed. ---> Xtensive.Core.AggregateException: Validation failed. ---> Xtensive.Core.AggregateException: A set of exceptions is caught. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Reflection.AmbiguousMatchException: Ambiguous match found.

at System.RuntimeType.GetPropertyImpl(String name, BindingFlags bindingAttr, Binder binder, Type returnType, Type[] types, ParameterModifier[] modifiers) at System.Type.GetProperty(String name) at Xtensive.Reflection.DelegateHelper.InnerCreateGetMemberDelegateTObject,TValue,TDelegateType at Xtensive.Reflection.DelegateHelper.CreateGetMemberDelegateTObject,TValue at Xtensive.Orm.Validation.PropertyConstraintAspect.GetPropertyGetterTOwner,TProperty --- End of inner exception stack trace --- at System.RuntimeMethodHandle._InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeType typeOwner) at System.RuntimeMethodHandle.InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeType typeOwner) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at Xtensive.Orm.Validation.PropertyConstraintAspect.<getpropertyvalue>b__1(PropertyConstraintAspect _this) at Xtensive.Threading.ThreadSafeCached1.GetValue[T1](Func2 generator, T1 argument) at Xtensive.Orm.Validation.PropertyConstraintAspect.GetPropertyValue(IValidationAware target) at Xtensive.Orm.Validation.PropertyConstraintAspect.Check(IValidationAware target) at Xtensive.Core.ExceptionAggregator.ExecuteT --- End of inner exception stack trace --- at Xtensive.Core.ExceptionAggregator.Dispose() at Xtensive.Orm.Validation.ValidationAwareExtensions.CheckConstraints(IValidationAware target) at Xtensive.Orm.Persistent.InnerOnValidate() at Xtensive.Orm.Persistent.Xtensive.Orm.Validation.IValidationAware.OnValidate() at Xtensive.Orm.Validation.ValidationContext.Validate() --- End of inner exception stack trace --- at Xtensive.Orm.Validation.ValidationContext.Validate() at Xtensive.Orm.Transaction.CompleteValidation() --- End of inner exception stack trace --- at Xtensive.Orm.Transaction.CompleteValidation() at Xtensive.Orm.Transaction.Commit() at Xtensive.Orm.TransactionScope.Dispose() ...