title: Specifying column name in a "references" migration
body: <p>I want to make a <code>migration</code> in Rails, referencing another table. Usually, I would do something like:</p>
  
  <pre><code>add_column :post, :user, :references
  </code></pre>
  
  <p>This creates a column named <code>user_id</code> in <code>posts</code> table. But what if, instead of <code>user_id</code>, I want something like <code>author_id</code>? How can I do that?</p>