title: how can I save a form with ModelMultipleChoiceField?
body: <p>I have a model Calendar and in a form I want to be able to create multiple instances of it.</p>
  
  <p>Here are my models:</p>
  
  <pre><code>class Event(models.Model):
      user = models.ForeignKey(User)
  
  class Group(models.Model):
      name = models.CharField(_('Name'), max_length=80)
      events = models.ManyToManyField(Event, through='Calendar')
  
  class Calendar(models.Model):
      event = models.ForeignKey(Event)
      group = models.ForeignKey(Group)
  
  class CalendarInline(admin.TabularInline):
      model = Calendar
      extra = 1
  
  class GroupAdmin(admin.ModelAdmin):
      inlines = (CalendarInline,)
  </code></pre>
  
  <p>Here is how I try to code my form:</p>
  
  <pre><code>class AddEventToGroupForm(ModelForm):
      group = ModelMultipleChoiceField(queryset=Group.objects.all(), widget=SelectMultiple())
  
      def save(self):
          for g in self:
              g.save()
  
      class Meta:
          model = Calendar
          fields = ('group',)
  </code></pre>
  
  <p>And here is a part of my view:</p>
  
  <pre><code>e = Event.objects.get(id=event_id)
  calentry = Calendar(event=e)
  if request.POST:
      f = AddEventToGroupForm(data=request.POST, instance=calentry)
      if f.is_valid():
          f.save()
  </code></pre>
  
  <p>If I try to submit that form, I get:</p>
  
  <pre><code>AttributeError at /groups/add_event/7/
  
  'BoundField' object has no attribute 'save'
  </code></pre>
  
  <p>What is the proper way to create multiple instances of Calendar in this
  situation?</p>