title: Can't save model using F expression
body: <p>I'm using <em>F expressions</em> to update some of my model fields.
  I want to do it using specific custom model methods, but it's not working.</p>
  
  <p>This is my model</p>
  
  <pre><code>class SingletonModel(models.Model):
  
      class Meta:
          abstract = True
  
      def save(self, *args, **kwargs):
          self.__class__.objects.exclude(pk=self.pk).delete()
          super().save(*args, **kwargs)
  
      @classmethod
      def load(cls):
          try:
              return cls.objects.get()
          except cls.DoesNotExist:
              return cls()
  
  class Folios(SingletonModel):
      _folios = models.PositiveIntegerField(default=0, db_column='folios')
      fecha_ultima_compra = models.DateTimeField()
      fecha_ultima_venta = models.DateTimeField()
  
      def agregar_folios(self, valor):
          self.fecha_ultima_compra = timezone.now()
          self._folios=models.F('folios') + valor
          self.save()
  
      def sustraer_folios(self, valor):
          self.fecha_ultimo_venta = timezone.now()
          self._folios=models.F('folios') - valor
          self.save()
  </code></pre>
  
  <p>And when I do</p>
  
  <pre><code>f = Folios.load()
  f.agregar_folios(200)
  </code></pre>
  
  <p>I get the Traceback</p>
  
  <pre><code>Traceback (most recent call last):
    File "&lt;console&gt;", line 1, in &lt;module&gt;
    ... Some more info
      return int(value)
  TypeError: int() argument must be a string or a number, not 'CombinedExpression'
  </code></pre>