title: Add a column in a Rails Active Record migration with an initial value based on other columns
body: <p>I am attempting to add a column to an existing table using an Active Record migration in Rails.  I need the column's initial value to be based on other columns in the table.  Is there a way to do this in an Active Record migration?</p>
  
  <p>To make it more concrete, let's say I have the following:</p>
  
  <pre><code>my_table
  ----------------
  first_name: text
  last_name: text
  </code></pre>
  
  <p>I want to add a <code>full_name</code> text column with an initial value of <code>concat(first_name, ' ', last_name'</code>.  Note that I don't want a default value on the column, as I intend for the application to be populating this going forward (the initial default is just to have a sensible starting value for existing records).</p>
  
  <p>How can I do this in a migration?  Ideally I would like to use <code>add_column</code> or similar, though if that can't work a working alternative would be acceptable.</p>
  
  <p>Note that there already exists a nearly identical question (<a href="https://stackoverflow.com/questions/15284495">add a database column with Rails migration and populate it based on another column</a>), but none of its answers seem to fully answer this question.</p>