shipments_controller.rb

def show
  authorize! :show, @shipment
  choose_view_based_on_feature_flag
  @redirect_path = shipment_path(@shipment)
  @shipment_presenter = ShipmentShowPresenter.new(@shipment)
  respond_to do |format|
    format.html
  end
end

private
def choose_view_based_on_feature_flag
  @old_styles = FeatureGuard.shipment_show_v2? ? false : true
  @partial = FeatureGuard.shipment_show_v2? ? "shipments/v2/shipment" : "shipments/shipment"
end