title: EF 4.1 Bidirectional one-to-one problem
body: <p>Hi I am having a problem with a simple EF 4.1 code first model.</p>
  
  <p>I have a class person and a class survey that are bidirectionally linked. The database model is correct but I always get this error:</p>
  
  <pre><code>Unable to determine the principal end of an association between the types 'DAL.Models.Survey' and 'DAL.Models.Person'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.
  </code></pre>
  
  <p>Class Person</p>
  
  <pre><code>[Key]
          public Guid Id { get; set; }
          [Required]
          public string FirstName { get; set; }
          [Required]
          public string LastName { get; set; }
          [Required]
          public string Email { get; set; }
  
          public virtual Survey Survey { get; set; }
  </code></pre>
  
  <p>Class Survey</p>
  
  <pre><code>   [Key]
          public Guid Id { get; set; }
  
          public bool IsFinished { get; set; }
  
          public virtual Person Person { get; set; }
  </code></pre>
  
  <p>Datacontext:</p>
  
  <pre><code> modelBuilder.Entity&lt;Survey&gt;().HasRequired(s =&gt; s.Person).WithOptional().WillCascadeOnDelete(true);
  </code></pre>
  
  <p>Can anyone help please</p>