def minimum_needs_unit(field, feature, parent):
    """Retrieve units of the given minimum needs field name.

    For instance:
    * minimum_needs_unit('minimum_needs__clean_water') -> 'l/weekly'
    """
    _ = feature, parent  # NOQA
    field_definition = definition(field, 'field_name')
    if field_definition:
        unit_abbreviation = None
        frequency = None
        if field_definition.get('need_parameter'):
            need = field_definition['need_parameter']
            if isinstance(need, ResourceParameter):
                unit_abbreviation = need.unit.abbreviation
                frequency = need.frequency
        elif field_definition.get('unit'):
            need_unit = field_definition.get('unit')
            unit_abbreviation = need_unit.get('abbreviation')

        if field_definition.get('frequency') and not frequency:
            frequency = field_definition.get('frequency')

        if not unit_abbreviation:
            unit_abbreviation = exposure_unit['plural_name']

        once_frequency_field_keys = [
            'minimum_needs__toilets_count_field'
        ]
        if not frequency or (
                field_definition['key'] in once_frequency_field_keys):
            return unit_abbreviation.lower()

        unit_format = '{unit_abbreviation}/{frequency}'
        return unit_format.format(
            unit_abbreviation=unit_abbreviation, frequency=frequency).lower()

    return None
Retrieve units of the given minimum needs field name.

    For instance:
    * minimum_needs_unit('minimum_needs__clean_water') -> 'l/weekly'