[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, Inherited = true)]
public sealed class NoForeignKeyAttribute :
    Attribute

<AttributeUsage(AttributeTargets.Property Or AttributeTargets.Field, Inherited:=True)>
Public NotInheritable Class NoForeignKeyAttribute
    Inherits Attribute

public class Person : XPObject
{
    // ...
    [Association("Person-Orders")]
    public XPCollection<Order> Orders {
        get { return GetCollection<Order>(nameof(Orders)); }
    }
}

public class Order : XPObject {
    // ...
    [Association("Person-Orders"), NoForeignKey]
    public Person PersonID {
        get { return fPersonID; }
        set { SetPropertyValue(nameof(PersonID), ref fPersonID, value); }
    }
    Person fPersonID;

}

Class Person
   Inherits XPObject
   ' ...
   <Association("Person-Orders")> _
   Public ReadOnly Property Orders() As XPCollection(Of Order)
      Get
         Return GetCollection(Of Order)(NameOf(Orders))
      End Get
   End Property
End Class

Class Order
   Inherits XPObject
   ' ...
   <Association("Person-Orders"), NoForeignKey()> 
   Public Property Person() As PersonID
       Get
           Return fPerson
       End Get
       Set(ByVal value as PersonID)
           SetPropertyValue(NameOf(Person), fPerson, value)
       End Set
   End Property
   Private fPerson As PersonID

End Class