title: Modify models field via view doesn't work
body: <p>In my Django application, I want to subtract 1 "free_places" field in the "Event" model using the "EventDetailView" view where the form is located. Each time the form is OK (when the user subscribes to the event), the "free_places" field should decrease by 1.</p>
  
  <p>I do not know why my code does not work.</p>
  
  <p>My view:</p>
  
  <pre><code>class EventDetailView(DetailView, ModelFormMixin):
      model = models.Event
      form_class = forms.RegisterForm
      context_object_name = 'event'
  
      def get_success_url(self):
          return reverse('events:list')
  
      def get_initial(self):
          return {'event': self.kwargs['pk']}
  
      def post(self, request, *args, **kwargs):
          form = self.get_form()
          if form.is_valid():
              self.object = self.get_object()
              self.object.free_places - 1
              self.object.save()
              return self.form_valid(form)
          else:
              return self.form_invalid(form)
  </code></pre>
  
  <p>Models:</p>
  
  <pre><code>class Event(models.Model):
      title = models.CharField(max_length=500)
      date = models.DateField()
      text = models.TextField()
      image = FilerImageField(null=True, blank=True)
      flag = models.ForeignKey(Flag)
      free_places = models.IntegerField()
  
      class Meta:
          ordering = ['-date']
  
      def __str__(self):
          return self.title
  
      @property
      def slug(self):
          return slugify(self.title)
  
      def get_absolute_url(self):
          return reverse('events:detail', args=[self.slug, self.id])
  
      def get_model_id(self):
          return self.id
  
  
  class Register(models.Model):
      event = models.ForeignKey(Event)
      first_name = models.CharField(max_length=30)
      last_name = models.CharField(max_length=30)
      company = models.CharField(max_length=30, blank=True)
      street = models.CharField(max_length=50, blank=True)
      post_code = models.CharField(max_length=30, blank=True)
      city = models.CharField(max_length=30, blank=True)
      email = models.EmailField()
      phone_number = models.IntegerField()
  
      def __str__(self):
          return self.first_name
  
      def get_event_name(self):
          return self.event
  </code></pre>