title: Setting default username in Rails
body: <p>I am using devise gem for user management. So, I have a User model.</p>
  <p>What I want to be able to do is have a username column in the User model. By default, I want to be able to set the default username for users as 'user'+id where id is unique for every user and a column in User model.</p>
  <p>I want to be able to do something like:</p>
  <pre><code>class AddColsToUser &lt; ActiveRecord::Migration[6.1]
  
    def change
      add_column :users, :username, :string, default: 'user'+id
    end
  
  end
  </code></pre>
  <p>Is that possible? Thanks for reading :))</p>