title: Automatically generate new migration
body: <p>So assume I have used rails generate to create a model and subsequently it created migrations for it.</p>
  
  <p>Now it turns out I made a couple of modifications to those models (fixed up some relations). Is it possible to create migrations based off of my current model?</p>
  
  <ul>
  <li>I don't want to use rails generate to generate the model again.</li>
  <li>I don't want to manually write a migration.</li>
  </ul>