title: EF Parent-Child relationship where ChildID in Parent could be null
body: <p>I have an entity "Parent" that includes id of an entity "Child" that includes id to entity "Parent" as parent.
  For entity "Parent" more than one entity "Child" may be possible.
  "ChildID" of Entity "Parent" may is null in db. I.e. no "Child" current selected.
  If ChildID of entity "Parent" is null, EF should not does mapping for this field.</p>
  
  <p>simplified:</p>
  
  <pre><code>class Parent
  {
      public int ID {get; set;}
      public string Name {get; set;}
      public int? ChildID
      public Child Child {get; set;}
  }
  
  class Child
  {
      public int ID {get;set;}
      public string Name {get;set;}
      public int ParentID{get;set;}
      [Required]
      public Parent Parent {get;set;}
  }
  </code></pre>
  
  <p>When i launch EF did a mapping is abracadabra.</p>
  
  <p>I redesigned the db to added a new table contains ParentID and ChildID.</p>
  
  <p>How to solve this competently? Would be without addition table.</p>