title: How can I set a foreign key to allow nulls using ServiceStack OrmLite?
body: <p>I am using ServiceStack v4.x VS2013</p>
  
  <p>By default ServiceStack ORMLite (SqlServer) defines foreign keys with "NOT NULL".
  The following code produces a foreign key "FooId (FK, long, not null)"
  How can I tell ServiceStack this foreign key may be null?</p>
  
  <pre><code>public class Blah
  {
      [AutoIncrement]
      public long Id { get; set; }
      public string Name { get; set; }
  
      [References(typeof(Foo))]
      public long FooId { get; set; }
  
  }
  
  public class Foo
  {
      [AutoIncrement]
      public long Id { get; set; }
      public string Description { get; set; }
  }
  </code></pre>