title: Delete database rows that don't fit new criteria (in ruby on rails)
body: <p>I have a rails application where a "profile" has a has_many association with another model, where the other model has one primary defining attribute that is selected via drop-down. I'm doing an update where the values in the drop-down are going to be changed and I want the underlying data in the database to reflect this change.</p>
  
  <p>When I deploy to the server, what is an easy/clean way to delete any pre-existing database entries for items that no longer are included in the drop-down list (I have the drop-down list data accessible in an array)? Is there any way to do it in a migration (or would it even make sense if I'm not changing any actual database structure)?</p>
  
  <p>I'm not overly familiar with rails, so I hope this is clear!</p>