title: ASP Entity Framework Fluent API [ForeignKey] annotation error
body: <p>I'm trying to set up a one to one relationship and ran into some issue with declaring an attribute as FK.  I already searched and read some of the questions posted here but doesn't address my problem.</p>
  
  <pre><code>public class User
  {
      [Key]
      public int userId {get;set;}
      [DisplayName("User Name")]
      [Required(ErrorMessage="User name required.")]
      public string username {get;set;}
      [DisplayName("Password")]
      [Required(ErrorMessage="Password required.")]
      [MinLength(6)]
      public string password {get;set;}
      [DisplayName("Email")]
      [Required(ErrorMessage="Email required.")]
      public string email {get;set;}
  
      public virtual List&lt;RoleDetail&gt; roleDetails { get; set; }
      public virtual Customer customer { get; set; }
  }
  
  public class Customer
  {
      [Key]
      public int cusomterId { get; set; }
      [DisplayName("First Name")]
      [Required(ErrorMessage="First name required.")]
      public string firstname {get;set;}
      [DisplayName("Last Name")]
      [Required(ErrorMessage="Last name required.")]
      public string lastname {get;set;}
      [ForeignKey("userId")]
      public int userId {get;set;}
  }
  </code></pre>
  
  <p>I'm getting this error when I use the [ForeignKey] annotation.  And I'm using <strong>System.ComponentModel.DataAnnotations</strong>.  Also, [Key] works fine.</p>
  
  <pre><code>The type or namespace name 'ForeignKeyAttribute' could not be 
  found (are you missing a using directive or an assembly reference?) 
  </code></pre>
  
  <p>What am I missing here?</p>