title: The type or namespace name 'DbContext' could not be found
body: <p>I am VERY new to ASP.NET MVC (3) and am having a hard time resolving a build error in Visual Studio:</p>
  
  <p>The type or namespace name 'DbContext' could not be found (are you missing a using directive or an assembly reference?)</p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Web;
  using System.ComponentModel.DataAnnotations;
  using System.Data.Entity;
  
  namespace MyProjectName.Models
  {   
      public class MachineModel
      {
          // name
          [Required]
          [Display(Name = "Nom de la machine")]
          public string Name { get; set; }
  
          // IP
          [Required]
          [RegularExpression(@"(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)",
             ErrorMessage = "Donnez une adresse IPv4 valide.")]
          [Display(Name = "Adresse IP de la machine")]
          public string IP { get; set; }
      }
  
      public class MachineDbContext : DbContext
      {
          public DbSet&lt;MachineModel&gt; Machines{ get; set; }
      }
  }
  </code></pre>
  
  <p>The two errors I am getting are:</p>
  
  <ul>
  <li>The type or namespace name
  'DbContext' could not be found (are
  you missing a using directive or an
  assembly reference?) </li>
  <li>The type or namespace name 'DbSet'
  could not be found (are you
  missing a using directive or an
  assembly reference?)</li>
  </ul>
  
  <p>What am I missing?</p>