title: The type or namespace name 'Key' could not be found
body: <p>The [Key] attribute for entity framework is not working for me.  I have a <code>console</code> application and I am trying to add this class:</p>
  
  <pre><code>public class Post
  {
      [Key]
      public int IDPost { get; set; }
      public string Title { get; set; }
      public string Content { get; set; }
      public int IDBlog { get; set; }
      public virtual Blog Blog { get; set; }
  }
  </code></pre>
  
  <p>I get these errors:</p>
  
  <blockquote>
    <p>The type or namespace name 'Key' could not be found (are you missing a using directive or an assembly reference?)  </p>
    
    <p>The type or namespace name 'KeyAttribute' could not be found (are you missing a using directive or an assembly reference?) </p>
  </blockquote>
  
  <p>I have added this using:</p>
  
  <pre><code>using System.Data.Entity;
  </code></pre>
  
  <p>And I have these references added to the project:</p>
  
  <pre>
  EntityFramework
  EntityFramework.SqlServer
  System.ComponentModel.DataAnnotations
  </pre>
  
  <p>I am using EF6 and VS2012.</p>
  
  <p>What reference am I missing?</p>