title: Why doesn't sqlalchemy modify the database when I add/remove a column?
body: <p>If I run my python script, it creates the tables correctly.</p>
  
  <p>However, if I add/remove a column and run the script again, it doesn't modify the database.</p>
  
  <p>Why is this?</p>
  
  <p>I am doing this after defining everything:</p>
  
  <pre><code>mapper(Product, products)
  mapper(Category, categories)
  
  metadata.create_all(engine)
  
  session = Session()
  </code></pre>