title: Find a record in dbSet using Find without a primary key
body: <p>I have a users table:</p>
  
  <pre><code>Users:
   +ID
   +Username
   +...
  </code></pre>
  
  <p>I want to use <code>myDBContext.Users.Find(Username)</code> to fin a users.
  in my current context I can not use his ID.</p>
  
  <p>do i have to use a full LINQ query ? e.g.</p>
  
  <pre><code>var user = from users in myDBContext.Users.Find(Username) 
             where users.Username == username
             select users
  </code></pre>
  
  <p>I have also tried to define the username as a primary key in my edmx but that resulted in the following error:</p>
  
  <blockquote>
    <p>Properties referred by the Principal Role User must be exactly
    identical to the key of the EntityType
    CamelotShiftManagementModel.User referred to by the Principal Role in
    the relationship constraint for Relationship
    CamelotShiftManagementModel.AssociationUserFK1. Make sure all the key
    properties are specified in the Principal
    Role. C:\Code\CamelotShiftManagement\CamelotShiftManagement\Models\CamelotDB.edmx 278 11  CamelotShiftManagement</p>
  </blockquote>