Test case for https://github.com/rails/rails/issues/19570

rails_19570.rb

module Validators
  extend ActiveSupport::Concern

  class CountValidator < ActiveModel::EachValidator

    def initialize(options)
      if range = (options.delete(:in) || options.delete(:within))
        options[:minimum] = range.min
        options[:maximum] = range.max
      end
      super
    end

    def validate_each(record, attribute, value)
      count = select(value).size # Delivery use size to prevent extra queries
      if options.has_key?(:minimum) && count < options[:minimum]
        record.errors.add attribute, :too_few, count: options[:minimum]
      elsif options.has_key?(:maximum) && count > options[:maximum]
        record.errors.add attribute, :too_many, count: options[:maximum]
      elsif options.has_key?(:is) && count != options[:is]
        record.errors.add attribute, :wrong_count, count: options[:is]
      end
    end

    private

    def select(value)
      if options[:selector]
        options[:selector].call(value)
      end || value
    end

  end
  module ClassMethods

    def validates_count_of(*attr_names)
      validates_with CountValidator, _merge_attributes(attr_names)
    end

  end
end