title: django-storages get the full S3 url
body: <p>I have a couple of classes using django-storages with Amazon S3</p>
  
  <pre><code>class Cache(models.Model):
      identifier = models.TextField(blank=True, null=True)
      cache_file = models.FileField(upload_to="cache")
  </code></pre>
  
  <p>Now I need to get the url for the cache file location. </p>
  
  <pre><code>cache = cache.objects.get(identifier=identifier)
  cache_file = cache.cache_file
  </code></pre>
  
  <p>cache file is a <code>FieldFile</code> object which contains a <code>storage</code> object. </p>
  
  <p>in the database I only see the value <code>cache/file.json</code> which I saved earlier.</p>
  
  <p>In this case I do not need to get the file, but the full url where the file is located.<br>
  How would I be able to get this?</p>