title: Django-tables2 column total
body: <p>I'm trying to sum up all values from column using this <a href="https://github.com/bradleyayers/django-tables2/blob/master/docs/pages/column-headers-and-footers.rst" rel="nofollow">documentation</a>, but footer doesn't show up. I'm I missing something?</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class Mokejimai(models.Model):
      id = models.AutoField(primary_key=True)
      nr = models.IntegerField(verbose_name='Mok. Nr.')
      data = models.DateField(verbose_name='Kada sumokėjo')
      suma = models.FloatField(verbose_name='Sumokėta suma')
      skola_pagal_agnum = models.FloatField(verbose_name='Skola pagal Agnum')
      date_entered = models.DateTimeField(auto_now_add=True, auto_now=False, verbose_name='Apmokėjimas įvestas')
      date_modified = models.DateTimeField(auto_now_add=False, auto_now=True, blank=True, null=True)
      imone = models.ForeignKey(Imones, models.DO_NOTHING, verbose_name='Įmonė')
      sask = models.ForeignKey(Saskaitos, blank=True, null=True, verbose_name='Sąskaita')
      user = models.ForeignKey(User, models.DO_NOTHING, default=settings.AUTH_USER_MODEL)
  </code></pre>
  
  <p><strong>tables.py</strong></p>
  
  <pre><code>class MokejimaiTable(tables.Table):
      suma = tables.Column(footer=lambda table: sum(x['suma'] for x in table.data))
  
      class Meta:
          model = Mokejimai
          attrs = {"class": "paleblue"}
          fields = ('id', 'imone', 'sask', 'nr', 'suma', 'skola_pagal_agnum', 'data', 'date_entered')
  </code></pre>