code: def _build_query_dict(self, formdata=None):
          
          if self.is_valid() and formdata is None:
              formdata = self.cleaned_data
          key = "{field}__{operator}".format(**formdata)
          if formdata['operator'] == "isnull":
              return {key: None}
          elif formdata['operator'] == "istrue":
              return {formdata['field']: True}
          elif formdata['operator'] == "isfalse":
              return {formdata['field']: False}
          return {key: formdata['value']}
docs: Take submitted data from form and create a query dict to be
          used in a Q object (or filter)