title: Django check if object in ManyToMany field
body: <p>I have quite a simple problem to solve. I have Partner model which has &gt;= 0 Users associated with it:</p>
  <pre><code>class Partner(models.Model):
      name = models.CharField(db_index=True, max_length=255)
      slug = models.SlugField(db_index=True)
      user = models.ManyToManyField(User)
  </code></pre>
  <p>Now, if I have a User object and I have a Partner object, what is the most Pythonic way of checking if the User is associated with a Partner? I basically want a statement which returns True if the <code>User</code> is associated to the <code>Partner</code>.</p>
  <p>I have tried:</p>
  <pre><code>users = Partner.objects.values_list('user', flat=True).filter(slug=requested_slug)
  if request.user.pk in users:
      # do some private stuff
  </code></pre>
  <p>This works but I have a feeling there is a better way. Additionally, would this be easy to roll into a decorator, baring in mind I need both a named parameter (<code>slug</code>) and a request object (<code>user</code>).</p>