Rails custom validation using answer_before_type_cast

rails_answer_before_type_cast.rb

class Answer < ActiveRecord::Base

  validate :check_answer_validity

  def check_answer_validity
    raw_answer = self.answer_before_type_cast
    answer_type = Question.find($question_id).answer_type

    # first test the answer against the corresponding regex
    case answer_type
      when "# Attended", "# of Occurrences"
        # regex allows "#" as valid match
        show_error = /\A\d+\z/ === raw_answer ? false : true
      when "0=No, 1=Yes"
        # regex allows "0" or "1" as valid matches
        show_error = /\A(0|1)\z/ === raw_answer ? false : true
      when "Percent"
        # regex allows "#", "#.#", "#.", or ".#" as valid matches
        show_error = /\A(\d+([.]|)(\d+|)|(\d+|)([.]|)\d+)\z/ === raw_answer ? false : true
    end

    # then construct and show the error based on the answer test
    if show_error
      error = "^'#{raw_answer}' is an invalid entry for "
      case answer_type
        when "# Attended"
          error += "'# Attended', you must enter a whole number, decimals are not allowed."
        when "# of Occurrences"
          error += "'# of Occurrences', you must enter a whole number, decimals are not allowed."
        when "0=No, 1=Yes"
          error += "'0=No, 1=Yes', you must enter only 0 for No or 1 for Yes."
        when "Percent"
          error += "'Percent', you must enter a whole number or decimal without the percent sign."
      end
      self.errors[:base] << error
    end
  end

end