return filtering params

def filter_params(self, value):
        """ return filtering params """
        if value is None:
            return {}

        val_min = value.get('min', None)
        val_max = value.get('max', None)
        params = {}

        if val_min == val_max:
            return { self.target: val_min }

        key = self.target + "__"
        if val_min is not None:
            params[key+self.lookup_types[0]] = val_min
        if val_max is not None:
            params[key+self.lookup_types[1]] = val_max

        return params