code: function(model, logger) {
      // This line has to come first
      validationHelper.validateModel(model, logger)
      const Log = logger.bind()
  
      let sortableFields = this.getReadableFields(model, Log)
  
      for (let i = sortableFields.length - 1; i >= 0; i--) {
        let descendingField = '-' + sortableFields[i]
        sortableFields.splice(i, 0, descendingField)
      }
  
      return sortableFields
    }
docs: Get a list of valid query sort inputs.
  @param model: A mongoose model object.
  @param logger: A logging object.
  @returns {Array}: A list of fields.