title: How to initialize ModelMultipleChoiceField from table value?
body: <p>Django 1.4, I am using ModelMultipleChoiceField in an admin page, so I have added to form like below</p>
  
  <pre><code>class EmailBatchForm(ModelForm):
     proposals = forms.ModelMultipleChoiceField(required=False,\
        widget=FilteredSelectMultiple("proposals", is_stacked=False),\
        queryset=BuyRequest.objects.all())
  
  def __init__(self, *args, **kwargs):
  </code></pre>
  
  <p>It saves to table with value string like 
    "[, ]"</p>
  
  <p>When I edit the record, I need the ModelMultipleChoiceField populated with the save value, it seems I have to manually set it</p>
  
  <pre><code>def __init__(self, *args, **kwargs):
      super(EmailBatchForm, self).__init__(*args, **kwargs)
      self.initial['proposals'] = BuyRequest.objects.filter(...)  
  </code></pre>
  
  <p>I have to give it a queryset, but how do I filter the queryset with the saved string value?</p>
  
  <p>The saved string value is just pair of Object type and object name, seems it cannot unique identify each record, I think a comma separated object key is better, but how can I set so it saves like that?</p>
  
  <p>Thanks</p>