title: Symfony unitOfWork
body: <p>I have a little problem to solve.</p>
  <p>I have two entities with a relation many to mamy (something like posts and tags)
  I want to have a event that is called onFlush on post entity.
  The case is that on this event I must have the changes that are made on a collection od tags to that post and the base collection before the changes are made.
  The first part i know how to make by getScheduledCollectionUpdates but how to get the base collection of the entity that will be updated?</p>
  <p>Im using symfony 4.4 and doctrine</p>