'Get the filter field value used for \'server\' type filters.
This is the value from the filter action\'s list of filter choices.'

def get_filter_field(self):

    filter_action = self._meta._filter_action
    param_name = ('%s_field' % filter_action.get_param_name())
    filter_field = self.request.session.get(param_name, '')
    return filter_field
