module ActiveRecord::Aggregations::ClassMethods

def writer_method(name, class_name, mapping, allow_nil, converter)
  define_method("#{name}=") do |part|
    klass = class_name.constantize
    unless part.is_a?(klass) || converter.nil? || part.nil?
      part = converter.respond_to?(:call) ? converter.call(part) : klass.send(converter, part)
    end
    hash_from_multiparameter_assignment = part.is_a?(Hash) &&
      part.keys.all?(Integer)
    if hash_from_multiparameter_assignment
      raise ArgumentError unless part.size == part.each_key.max
      part = klass.new(*part.sort.map(&:last))
    end
    if part.nil? && allow_nil
      mapping.each { |key, _| write_attribute(key, nil) }
      @aggregation_cache[name] = nil
    else
      mapping.each { |key, value| write_attribute(key, part.send(value)) }
      @aggregation_cache[name] = part.dup.freeze
    end
  end
end