title: Implementing Zero Or One to Zero Or One relationship in EF Code first by Fluent API
body: <p>I have two POCO classes:</p>
  <p><strong>Order Class:</strong></p>
  <pre><code>public class Order
  {
      public int Id { get; set; }
      public int? QuotationId { get; set; }
      public virtual Quotation Quotation { get; set; }
      ....
  }
  </code></pre>
  <p><strong>Quotation Class:</strong></p>
  <pre><code>public class Quotation
  {
      public int Id { get; set; } 
      public virtual Order Order { get; set; }
      ....   
  }
  </code></pre>
  <ul>
  <li>Each <code>Order</code> <strong>may</strong> be made from  one or zero quotation, and</li>
  <li>each quotation <strong>may</strong> cause an order.</li>
  </ul>
  <p>So I have an &quot;one or zero&quot; to &quot;one or zero&quot; relation, how can I implement this, in <code>EF</code> Code first by <code>Fluent</code> API?</p>