dynamic_attach_to_spec.rb

    report_klass = Class.new do
      include Mongoid::Report

      report 'example' do
        columns :field4 => ->(context, row, options) { row['field3'] }

        column :field3, :field4

        attach_to 'pregenerated' do
          group_by :field1
        end

        attach_to Model do
          group_by :field1, :field2
        end
      end
    end