title: many-to-many relationship in doctrine
body: <p>I've got some tables on my system like these:</p>
  
  <pre><code>news
  --id
  --title
  --content
  
  video
  --id
  --title
  --url
  
  album
  --id
  --title
  </code></pre>
  
  <p>Now, I need to do a many-to-many relatioship with this tables, but in a flexible way. I created a table called 'links' with the below structure:</p>
  
  <pre><code>links
  --parent_entity (example: news)
  --parent_id   (example: 5)
  --child_entity (exmaple: video)
  --child_id (example: 2)
  </code></pre>
  
  <p>How can I map this using Doctrine?</p>