array_validator.rb

class ArrayValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    array_elements_are_unique(record, attribute, value)
    array_elements_are_subset_of_valid_list(record, attribute, value, options[:with])
    field_is_present(record, attribute, value)
  end

  def array_elements_are_unique(record, attribute, value)
    if value.present? && (value.uniq.size < value.size)
      record.errors.add(attribute, 'are duplicated')
      false
    else
      true
    end
  end

  def array_elements_are_subset_of_valid_list(record, attribute, value, valid_list)
    if (value & valid_list) == value
      true
    else
      record.errors.add(attribute, 'are not valid')
    end
  end

  def field_is_present(record, attribute, value)
    if !value.nil? && value.try(:size) >= 0
      true
    else
      record.errors.add(field_name, 'are missing')
      false
    end
  end
end

user.rb

class User
  VALID_RACES = %w[black americanIndian asian pacificIslander white]
  validates :races, array: { with: VALID_RACES }
end