func (m *Mapping) FilterFieldByName(name string) (*Field, error) {
	field := m.FieldByName(name)
	if field == nil {
		return nil, fmt.Errorf("Unknown filter %q", name)
	}

	if field.Type.Code != TypeColumn {
		return nil, fmt.Errorf("Unknown filter %q not a column", name)
	}

	return field, nil
}
func (m *Mapping) FilterFieldByName(name string) (*Field, error) {
	field := m.FieldByName(name)
	if field == nil {
		return nil, fmt.Errorf("Unknown filter %q", name)
	}

	if field.Type.Code != TypeColumn {
		return nil, fmt.Errorf("Unknown filter %q not a column", name)
	}

	return field, nil
}
// FilterFieldByName returns the field with the given name if that field can be
// used as query filter, an error otherwise.