title: generate a model with a string field as primary key
body: <p>I want to create a <code>User</code> model inside my Ruby on Rails application.</p>
  
  <p>I use following command:</p>
  
  <pre><code>rails generate model User email:string name:string role:string
  </code></pre>
  
  <p>It is possible to define the <code>email</code> as primary key with this command? Or I must modify the database migration file that I create with this command? And how?</p>