Rails redirect from old URL to a new one that using slug
module ApplicationHelper
  def product_path(product)
    unless product.category.nil?
      cat = product.category
      if cat.ancestry_depth.eql?(2)
        product_long_path cat.root, cat.parent, cat, product
      else
        product_short_path cat.parent, cat, product
      end
    end
  end
end

get "/(*root_id)_(*subcategory_id)_(*category_id)_(*id)" => "products#show", :as => :product_long
  get "/(*root_id)_(*category_id)_(*id)" => "products#show", :as => :product_short

def show
    @product = Product.find_by_slug params[:id]
    cat = @product.category

    raise NotFound unless cat.slug.eql?(params[:category_id]) and cat.root.slug.eql?(params[:root_id])
    raise NotFound if cat.ancestry_depth.eql?(2) and !cat.parent.slug.eql?(params[:subcategory_id])
  end

@product = Product.find_by_slug params[:id]
if @product.nil?
  @product = Product.find params[:id]
  redirect_to @product.redirect_string and return
end

def redirect_string
    if category.ancestry_depth.eql?(2)
      "/#{category.root.slug}_#{category.parent.slug}_#{category.slug}_#{slug}"
    else
      "/#{category.parent.slug}_#{category.slug}_#{slug}"
    end
  end