title: ManyToMany insertion issue
body: <h2>The goal</h2>
  
  <p>Create a new market and add its manager successfully.</p>
  
  <h2>The problem</h2>
  
  <blockquote>
    <p>Entity Validation Failed - errors follow:</p>
    
    <p>MyApp.Models.Data.Users failed validation</p>
    
    <ul>
    <li><p>Email : The Email field is required.</p></li>
    <li><p>PHash : The PHash field is required.</p></li>
    <li><p>PSalt : The PSalt field is required.</p></li>
    </ul>
  </blockquote>
  
  <h2>The scenario</h2>
  
  <p>I'm creating a market on my application and I can set to it a manager. In other words, I'm attaching to a market a user that already exists. I'm trying to do this by the following code:</p>
  
  <pre><code>[HttpPost]
  public ActionResult Add(Market market)
  {
      [...]
  
      Market marketBasics = new Market
      {
          Name = market.Name,
          Slug = market.Slug,
          Manager = market.ManagerId
      };
  
      [...]
  
      User user = new User
      {
          Id = market.ManagerId
      };
  
      db.Markets.Add(marketBasics); // Here I insert all the information
                                    // into "markets" table.
  
      marketBasics.User.Add(user);  // Here I (attempts to) insert into 
                                    // "users_in_markets"
  
      [...]
  }
  </code></pre>
  
  <p>As you can see, I'm not setting the user's <code>Email</code>, <code>PHash</code> or <code>PSalt</code> — I'm not registering a new user, but I'm associating an existing (user) to a (new) market.</p>
  
  <p>So I ask: <strong>How can I resolve this?</strong></p>
  
  <h2>Technical details</h2>
  
  <p>I'm using Entity Framework 5 + C# + MySQL.</p>