docs: def parse_validated_field(fld, selectable):
      """ Converts a validated field to sqlalchemy. Field references are
      looked up in selectable """
      aggr_fn = IngredientValidator.aggregation_lookup[fld['aggregation']]
  
      field = find_column(selectable, fld['value'])
  
      for operator in fld.get('operators', []):
          op = operator['operator']
          other_field = parse_validated_field(operator['field'], selectable)
          field = IngredientValidator.operator_lookup[op](field)(other_field)
  
      condition = fld.get('condition', None)
      if condition:
          condition = parse_condition(condition, selectable)
          field = case([(condition, field)])
  
      field = aggr_fn(field)
      return field