Model concern that allows the HABTM records creation faster with single SQL's INSERT command

fast_save_has_and_belongs_to_many.rb

# Note:
#   - Use fast_save_habtm_<singular>_ids for this feature
#   - Callbacks and validations will NOT be triggered
#   - Associations will be saved on after_save callback
#   - Use <singular>_ids for normal habtm saving
module Shared::FastSaveHasAndBelongsToMany
  extend ActiveSupport::Concern

  included do
    class_eval do
      class << self
        alias_method_chain :has_and_belongs_to_many, :fast_save_habtm
      end
    end
  end

  module ClassMethods

    def has_and_belongs_to_many_with_fast_save_habtm(name, scope = nil, options = {}, &extension)
      has_and_belongs_to_many_without_fast_save_habtm(name, scope, options, &extension)
      define_fast_save_habtm_methods(name, options)
    end

    private

      def define_fast_save_habtm_methods(name, options)
        method_name   = :"fast_save_habtm_#{name.to_s.singularize}_ids"
        callback_name = :"fast_save_habtm_#{name}"

        class_eval do
          attr_accessor method_name

          after_save callback_name, if: method_name

          define_method callback_name do
            send(name).delete_all
            ids = send(method_name).select(&:present?)
            if ids.present?
              connection              = self.class.connection
              foreign_key             = options[:foreign_key] || self.class.name.demodulize.underscore + "_id"
              association_foreign_key = options[:association_foreign_key] || name.to_s.singularize + "_id"
              join_table              = options[:join_table] || [self.class.table_name, name.to_s].sort.join("_")
              connection.execute "INSERT INTO #{connection.quote_table_name(join_table)} (#{connection.quote_column_name(foreign_key)}, #{connection.quote_column_name(association_foreign_key)}) VALUES " + ids.map { |n| "(#{id}, #{n.to_i})" }.join(", ")
            end
          end
        end
      end

  end

end