A question about field forms and the widget SelectMultiple

Hi everybody, I have a form with a field that uses the SelectMultiple widget:

class DocumentForm(forms.ModelForm):
   options = forms.CharField(label='Options', max_length=100 ,widget=forms.SelectMultiple(attrs={'class':'listbox'}, choices=([(aspect.id, unicode(aspect.title)) for aspect in aspects ])) , required=False)

How you can see the field is populated automacally from a list: "choices = ([(aspect.id, unicode(aspect.title)) for aspect in aspects ]))", this works wonderfull when I am inserting in the database the document model.

But when I want to make an update over the document object I use the same form and I populated the form like this:

myform = DocumentForm(instance= document)

I want that the listbox displayed with all the options available an the ones that are inserted in the database with the document mark as selected.
For instance if all the options available are A, B, C, D and the document I want to update has only the option A and C, I want in the list box options A, B , C ,  D but option A and C mark as selected.

Could you please help me do that ?
How would you do it ?

Thanks in advance.
Regards
Ariel

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