title: How do I add an identity property to a column?
body: <p>How do I make a migration that changes an existing column so that it is an identity? Meaning, I want that integer column to automatically increment.</p>
  
  <p>Is it something like:</p>
  
  <pre><code>add_identity :tablename, [:columnname]
  </code></pre>
  
  <p>It is not a primary key column.</p>