title: migrating from two ids to a polymorphic association
body: <p>I have a table that currently belongs to 2 different models. What I'd like to do is turn these into a polymorphic association instead.</p>
  
  <p>Currently the table <code>line_items</code> have a <code>cart_id</code> and an <code>order_id</code> now what I'd like to do is migrate both of these ids into a polymorphic one called <code>itemable_id</code> and fix the <code>itemable_type</code> to the correct one.</p>
  
  <p>How would I go about creating a migration for this, or what's the go-to solution for this case, where you have two ids columns and want to make it polymorphic?</p>