gistfile1.txt

require "bundler/setup"
require "active_record"
begin
  require "benchmark/ips"
rescue LoadError
  raise LoadError, "Run install `gem install benchmark-ips`"
end

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.connection.create_table(:orders) { |t| }
ActiveRecord::Base.connection.create_table(:items) { |t| t.references :order, null: false }

class Order < ActiveRecord::Base
  has_many :items
  accepts_nested_attributes_for :items
end

class Item < ActiveRecord::Base
end

class Order2 < ActiveRecord::Base
  self.table_name = :orders
  has_many :items, foreign_key: :order_id
  accepts_nested_attributes_for :items

  def assign_nested_attributes_for_collection_association(association_name, attributes_collection)
    options = nested_attributes_options[association_name]
    if attributes_collection.respond_to?(:permitted?)
      attributes_collection = attributes_collection.to_h
    end

    unless attributes_collection.is_a?(Hash) || attributes_collection.is_a?(Array)
      raise ArgumentError, "Hash or Array expected for attribute `#{association_name}`, got #{attributes_collection.class.name} (#{attributes_collection.inspect})"
    end

    check_record_limit!(options[:limit], attributes_collection)

    if attributes_collection.is_a? Hash
      keys = attributes_collection.keys
      attributes_collection = if keys.include?("id") || keys.include?(:id)
                                [attributes_collection]
                              else
                                attributes_collection.values
                              end
    end

    association = association(association_name)

    existing_records = if association.loaded?
                         association.target
                       else
                         attribute_ids = attributes_collection.filter_map { |a| a["id"] || a[:id] }
                         attribute_ids.empty? ? [] : association.scope.where(association.klass.primary_key => attribute_ids)
                       end

    existing_records = existing_records.index_by { |record| record.id.to_s }

    attributes_collection.each do |attributes|
      if attributes.respond_to?(:permitted?)
        attributes = attributes.to_h
      end
      attributes = attributes.with_indifferent_access

      if attributes["id"].blank?
        unless reject_new_record?(association_name, attributes)
          association.reader.build(attributes.except(*UNASSIGNABLE_KEYS))
        end
      elsif existing_record = existing_records[attributes["id"].to_s]
        unless call_reject_if(association_name, attributes)
          # Make sure we are operating on the actual object which is in the association's
          # proxy_target array (either by finding it, or adding it if not found)
          # Take into account that the proxy_target may have changed due to callbacks
          target_record = association.target.detect { |record| record.id.to_s == attributes["id"].to_s }
          if target_record
            existing_record = target_record
          else
            association.add_to_target(existing_record, skip_callbacks: true)
          end

          assign_to_or_mark_for_destruction(existing_record, attributes, options[:allow_destroy])
        end
      else
        raise_nested_attributes_record_not_found!(association_name, attributes["id"])
      end
    end
  end
end

Benchmark.ips do |x|
  order = Order.create
  Item.insert_all Array.new(2000, { order_id: order.id })
  items_attributes = order.items.map { |item| { id: item.id } }

  order = Order.first
  order.reload
  x.report("Order") do |times|
    order.update(items_attributes: items_attributes)
  end

  order2 = Order2.first
  order2.reload
  x.report("Order2") do |times|
    order2.update(items_attributes: items_attributes)
  end
end