Rails validator for Postgres decimal type with custom precision and scale

decimal_validator.rb

class DecimalValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    precision = options[:precision]
    scale = options[:scale]

    if !precision || !scale
      raise ArgumentError, "#{self.class.name} expects :precision and :scale option"
    end

    original_value = record.public_send("#{attribute}_before_type_cast").to_s

    real_wholes, real_decimals = original_value.split(".")
    wholes = (precision - scale) + (scale - (real_decimals.try(:length) || 0))

    regex = /\A\d{1,#{wholes}}(\.\d{0,#{scale}})?\z/

    if regex !~ original_value
      record.errors.add(attribute, :decimal_out_of_specs, precision: precision, scale: scale)
      return
    end

    # Prevent PG::NumericValueOutOfRange: ERROR: numeric field overflow
    # DETAIL: A field with precision 10, scale 4 must round to an absolute value less than 10^6.
    if value.round >= 10 ** (precision - scale)
      record.errors.add(attribute, :decimal_out_of_range, max: "10^#{precision - scale}")
    end
  end

end

decimal_validator_test.rb

require "test_helper"

class DecimalValidatorTest < ActiveSupport::TestCase

  # unfortunately needs a real model to test Postgres errors
  def valid_quantity?(quantity)
    record = build(:test_model, quantity: quantity)
    record.validate
    errors = record.errors[:quantity]
    errors.empty? && record.save
  end

  test "configuration matches testcases" do
    validator = TestModel.validators_on(:quantity).find { |v| v.kind_of?(DecimalValidator) }
    fail "Missing DecimalValidator" unless validator

    configuration = validator.options
    assert_equal 10, configuration[:precision]
    assert_equal 4, configuration[:scale]
  end

  test "accepts valid values" do
    assert valid_quantity?(0)
    assert valid_quantity?("0.0001")
    assert valid_quantity?("999999")
    assert valid_quantity?("123456.1234")
    assert valid_quantity?("999998.9999")
  end

  test "rejects invalid values" do
    refute valid_quantity?(nil)
    refute valid_quantity?("")
    refute valid_quantity?(".")
    refute valid_quantity?(".1")
    refute valid_quantity?("string")
    refute valid_quantity?(1_000_000)
    refute valid_quantity?("0.12345")
    refute valid_quantity?("1234567.1234")
  end

end