title: Make group by having count(*) with django orm
body: <p>With following model:</p>
  
  <pre><code>class Day(models.Model):
      date = models.DateField(
          unique=True,
          null=False,
          blank=False,
      )
  
  
  class Reservation(models.Model):
      user = models.ForeignKey(
          User,
          null=False,
          blank=False,
          related_name='reservations',
          verbose_name=_('Utilisateur'),
      )
      day = models.ForeignKey(
          Day,
          null=False,
          blank=False,
          related_name='reservations',
      )
  </code></pre>
  
  <p>To get days ids where number of reservations is lower or equal 5 i do:</p>
  
  <pre><code>SELECT day_id
  FROM meal_reservation
  GROUP BY day_id
  HAVING count(*) &lt;= 5
  </code></pre>
  
  <p>But how to do it with Django ORM ?</p>