title: Django ModelMultipleChoiceField and CheckboxSelectMultiple delete values
body: <p>I have a Form with:</p>
  
  <pre><code>somemodels = forms.ModelMultipleChoiceField(queryset=SomeModel.objects.none, widget=forms.CheckboxSelectMultiple, required=False)
  </code></pre>
  
  <p>I set my queryset with my __init__ function and and can even provide existing connections to my list on page load (existing objects are marked as checked).</p>
  
  <pre><code>    def __init__(self, *args, **kwargs):
         qs = kwargs.pop('SomeModel')
         inital_object = kwargs.pop('someothermodel')
         super(SelectDatesForm, self).__init__(*args, **kwargs)
         self.fields['timeframes'].queryset = qs
         selected_items = qs.filter(someothermodels=inital_object) #manytomany relationship
         self.fields['somemodels'].initial = selected_timframes
  </code></pre>
  
  <p>The issue I get is a combination of the fact that a) HTML only submits data for checked inputs and b) django forms remove all the already existing objects from "cleaned_data".</p>
  
  <p>When I submit the form, I get the <strong>cleaned_data</strong> list of my form only contains the newly selected values, not the ones that were already selected when the page was constructed.</p>
  
  <p>To delete the existing ones (i.e. if the user unselects a checkbox that was initially checked) I need to know either a) a total list of all selected values, or b) a list of values that have been unselected.</p>
  
  <p>Is there a smooth way of doing this without actually going through the original POST data?</p>