batch_create.rb

class BatchCreate
  class << self
    def perform(active_records:, conflict_fields:)
      return if active_records.empty?

      ActiveRecord::Base.connection.execute(
        <<-SQL.squish
          #{insert_statement(active_records.first)}
          #{values(active_records)}
          #{on_conflict_statement(active_records.first, conflict_fields)}
        SQL
      )
    end

    private

      INSERT_REGEX = /(INSERT INTO .* VALUES) (.*)/

      def insert_statement(active_record)
        sql = active_record.class.arel_table.create_insert.tap do |insert_manager|
          insert_manager.insert(
            active_record.send(:arel_attributes_with_values_for_create, active_record.attribute_names.sort)
          )
        end.to_sql
        INSERT_REGEX.match(sql).captures[0]
      end

      def on_conflict_statement(active_record, conflict_fields)
        return ';' if conflict_fields.blank?

        columns = active_record.attributes.keys.reject { |key| key.to_sym == :id }
        values = columns.map { |column| "EXCLUDED.#{column}" }.join(', ')
        "ON CONFLICT (#{conflict_fields.join(', ')}) DO UPDATE SET (#{columns.join(', ')}) = (#{values})"
      end

      def values_for(active_record)
        active_record.updated_at = Time.now if active_record.respond_to?(:updated_at)
        active_record.created_at ||= Time.now if active_record.respond_to?(:created_at)

        sql = active_record.class.arel_table.create_insert.tap do |insert_manager|
          insert_manager.insert(
            active_record.send(:arel_attributes_with_values_for_create, active_record.attribute_names.sort)
          )
        end.to_sql

        INSERT_REGEX.match(sql).captures[1]
      end

      def values(active_records)
        active_records.map { |active_record| values_for(active_record) }.join(",")
      end
  end
end