Rails Validator for Uniqueness Among Records Unmarked for Destruction

unmarked_for_destruction_uniqueness_validator.rb

class UnmarkedForDestructionUniquenessValidator < ActiveModel::EachValidator
  # validates that record.<attribute> is unique among all non-marked-for-
  # destruction 'siblings' of record, where 'siblings' are all objects
  # record.<parent>.<children> such that the object is not the current record
  #
  # If an option such as 'ignore_records_marked_for_destruction: true' existed
  # for the 'uniqueness' validator, this validator would do the same thing as:
  #   validates :attribute,
  #             uniqueness: {
  #               scope: :parent,
  #               ignore_records_marked_for_destruction: true
  #             }
  def validate_each(record, attribute, value)
    if options[:parent].nil?
      fail ArgumentError, ":parent cannot be nil"
    end
    if options[:children].nil?
      fail ArgumentError, ":children cannot be nil"
    end
    duped = record.send(options[:parent]).send(options[:children]).any? do |a|
      a != record && !a.marked_for_destruction? && a.send(attribute) == value
    end
    if duped
      record.errors[attribute] << (
        options[:message] || "has already been taken"
      )
    end
  end
end

USAGE.md

class Foo < ActiveRecord::Base
  belongs_to :bar, inverse_of: :foos
  validates :some_attribute,
            unmarked_for_destruction_uniqueness: {
                parent: :bar
              , children: :foos
              # , message: "Some message"
            }
end