title: Sonata Media: Change context programmatically
body: <p>I'm writing a little blog app where the user can publish public and private news. Users can attach files to these news. I have two contexts for this app: <code>public_news</code>, with files which can be accessed by everyone; and <code>private_news</code>, with files which can only be accessed if the user has log on.</p>
  
  <p>I want to be able to move files from the public_news context to the private_news context when the user changes a news from public to private, and vice versa.</p>
  
  <p>I was hoping to do something as simple as <code>$media-&gt;setContext('private_news');</code>, but this won't move the physical file from one directory to the other.</p>