class DensityValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    valid = false
    msg = 'is not a unit of density'

    begin
      unit = UnitConversion::parse(value)
      valid = UnitConversion::density? unit
    rescue UnitConversion::UnparseableUnitError => e
      valid = false
      msg = e.message
    end

    unless valid
      record.errors[attribute] << (options[:message] || msg)
    end
  end
end