Add bulk import functionality to Rails Active Record (add this file to config/initializers, call <model>.import!(array-of-record-hashes))

active_record.rb

# frozen_string_literal: true

# Makes BULK INSERT for ActiveRecord rows
class ActiveRecord::Base
  def import!(record_list, options = {})
    batch_size = options[:batch_size] || 1000
    raise ArgumentError "record_list not an Array of Hashes" unless valid_record_list?(record_list)
    return record_list if record_list.empty?

    record_list.each_slice(batch_size).each do |records|
      key_list, value_list = convert_record_list(records)
      sql = "INSERT IGNORE INTO #{table_name} (#{key_list.join(', ')}) VALUES #{joined_values_list(value_list)}"
      connection.execute(sql)
    end

    record_list
  end

  private

  def valid_record_list?(record_list)
    record_list.is_a?(Array) && record_list.all? { |rec| rec.is_a? Hash }
  end

  def joined_values_list(value_list)
    value_list.map { |rec| "(#{rec.join(', ')})" }.join(' ,')
  end

  def convert_record_list(record_list)
    key_list = record_list.map(&:keys).flatten.map(&:to_s).uniq.sort

    value_list = record_list.map do |rec|
      list = []
      key_list.each { |key| list << ActiveRecord::Base.connection.quote(rec[key] || rec[key.to_sym]) }
      list
    end

    time = ActiveRecord::Base.connection.quote(Time.current)
    %w[created_at updated_at].each do |field_name|
      if column_names.include?(field_name) && !key_list.include?(field_name)
        key_list << field_name
        value_list.each { |rec| rec << time }
      end
    end

    [key_list, value_list]
  end
end

Improved BULK INSERT from https://gist.github.com/jackrg/76ade1724bd816292e4e
Added batch_size as parameter and IGNORE into SQL
This gist was added because of absence the answers on the question 'Why 'activerecord-import' doesn't perform bulk insert?' was added this gist.
Also Rails 6 supports bulk insert