Faster pagination for kaminari

0_usage.rb

@payments_base = Payment.extending(FasterPagination)
@payments = @payments_base.page(params[:page]).per(20)

faster_pagination.rb

module FasterPagination
  include Kaminari::ActiveRecordRelationMethods
  include Kaminari::PageScopeMethods

  def total_count(column_name = :all, options = {})
    return @total_count_cached if @total_count_cached
    copy = except(:order, :includes)
    copy.select_values = ['1']

    # copy.limit_value * 5 + 1 => means count only next 6 pages
    # when call :count and qurey has "group by" it will return hash,
    # execute_simple_calculation - always return a number
    query_count = copy.limit(copy.limit_value * 5 + 1).send(:execute_simple_calculation, 'count', "1", false)
    total_count = copy.offset_value + query_count

    @total_count_cached = total_count
  end

  def per(num)
    if (n = num.to_i) <= 0
      self
    elsif max_per_page && max_per_page < n
      limit(max_per_page).offset(offset_value / limit_value * max_per_page)
    else
      limit(n).offset(offset_value / limit_value * n)
    end
  end

  def total_where_values
    defaults = default_scopes.map(&:call)
    (where_values + defaults.map(&:where_values)).flatten.uniq.map do |where_value|
      next if where_value.blank?
      where_value.is_a?(String) ? where_value : where_value.to_sql
    end
  end

  def total_order_values
    defaults = default_scopes.map(&:call)
    (order_values + defaults.map(&:order_values)).flatten.uniq.map do |order_value|
      next if order_value.blank?
      order_value.is_a?(String) ? order_value : order_value.to_sql
    end
  end

  eval <<-RUBY
    def #{Kaminari.config.page_method_name}(num = nil)
      limit(default_per_page).offset(default_per_page * ([num.to_i, 1].max - 1))
    end
  RUBY
end