title: Select a valid choice ModelChoiceField
body: <p>Whenever im running form.is_valid() i get the error: </p>
  
  <blockquote>
    <p>Select a valid choice. That choice is not one of the available
    choices.</p>
  </blockquote>
  
  <p>Here is what I do in my view:</p>
  
  <pre><code>timeframes = HostTimeFrame.objects.all()
  if request.method == 'POST':
      form = SelectDatesForm(request.POST, timeframes=timeframes)
      if form.is_valid():
          pass
  else:
      form = SelectDatesForm(timeframes=timeframes)
  </code></pre>
  
  <p>My form does this:</p>
  
  <pre><code> class SelectDatesForm(forms.Form):
      timeframes = forms.ModelChoiceField(queryset=HostTimeFrame.objects.none(), widget=forms.CheckboxSelectMultiple,
                                          empty_label=None)
      def __init__(self, *args, **kwargs):
          qs = kwargs.pop('timeframes')
          super(SelectDatesForm, self).__init__(*args, **kwargs)
          self.fields['timeframes'].queryset = qs.order_by('start')
  </code></pre>
  
  <p>Ive been trying for hours to find where this actual validation is done, and i found it, created a bug <a href="https://code.djangoproject.com/ticket/27578#ticket" rel="nofollow noreferrer">here</a>.</p>