code: def minimum_needs_unit(field, feature, parent):
      
      _ = 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
docs: Retrieve units of the given minimum needs field name.
  
      For instance:
      * minimum_needs_unit('minimum_needs__clean_water') -> 'l/weekly'