title: How to create a new not null column with existing data?
body: <p>I am creating a new column in a table which is a uuid/guid.</p>
  
  <p>add_column :users, :uuid, :string, null: false</p>
  
  <p>I am setting the uuid using:</p>
  
  <pre><code>SecureRandom.uuid
  </code></pre>
  
  <p>The migration fails because the existing data doesn't have any value.</p>
  
  <p>How can I set the value for each user with a new uuid during this migration?</p>