title: ModelMultipleChoiceField dosen't work correctly in django form
body: <p>I need a combobox  in my template. so I use ModelMultipleChoiceField when I try to submit my form It said this field is empty and you must choose !
  myform:</p>
  
  <pre><code>class SearchForm(forms.Form):
       type_pm = forms.ModelMultipleChoiceField(queryset=IvrModel.objects.all(), widget=forms.Select())
  </code></pre>
  
  <p>my template:</p>
  
  <pre><code>{% for field in form %}
  
       {{ field.label_tag }}
       {{ field }}
  {% endfor %}
  </code></pre>
  
  <p>how do I fix It?</p>