Faster Pagination in Rails

By on September 10, 2006

Pagination in Rails has recently surfaced as a hot topic thanks to Kevin Clark's "Thing's you shouldn't be doing in Rails". There seems to be a lot of confusion around the whole issue, no one has offered a definitive reason as to why the default Pagination helpers shouldn't be used, and at the same time there seems to be a consensus among the 'in' people that you should roll your own pagination in your apps. So what's wrong with the default pagination code?

Well, on first look, not much. If you use it correctly Rails will in fact limit your database queries to correct sizes and pages, so no database overhead here. What about the pagination helper itself? Window code? Yes, there does seem to be a problem here! Rails will by default instantiate a number of 'Page' classes and Window classes once you start using the more advanced features of the helper. Now, instantiating 100+ objects for generating 5 links is never a good idea, ok, point taken. But, I think the biggest problem is in fact not the helpers themselves but how the developers use the code. Some will retrieve an entire collection (find all) and then pass it to the paginator to extract the right collection (think paginate_collection), now we are incurring a double overhead - the database query and the additional load on generating the classes. In retrospect, it seems that the helpers are just too powerful, they make it to easy too write inefficient code, and as such it's not that they should be avoided, they should be understood!

Now, how do we solve the problem? Well, an elegant solution is to use something like: paginating_find. It's a drop-in plugin which will overwrite your default find method if you specify the page parameters in your find query. Now, the database side is taken care of, what about the presentation, after all some of those helpers were there for a good reason! Here is a quick extension/helper I came up with to replace the pagination_links_each to work with paginating_find:

 def windowed_pagination_links(pagingEnum, options)
    link_to_current_page = options[:link_to_current_page]
    always_show_anchors = options[:always_show_anchors]
    padding = options[:window_size]

    current_page = pagingEnum.page
    html = ''

    #Calculate the window start and end pages
    padding = padding < 0 ? 0 : padding
    first = pagingEnum.page_exists?(current_page  - padding) ? current_page - padding : 1
    last = pagingEnum.page_exists?(current_page + padding) ? current_page + padding : pagingEnum.last_page

    # Print start page if anchors are enabled
    html << yield(1) if always_show_anchors and not first == 1

    # Print window pages
    first.upto(last) do |page|
      (current_page == page && !link_to_current_page) ? html << page : html << yield(page)
    end

    # Print end page if anchors are enabled
    html << yield(pagingEnum.last_page) if always_show_anchors and not last == pagingEnum.last_page
    html
  end

It takes the Enum produced by paginating_find and via some simple math (no extra classes instantiated here!) returns control to your block for each link in your window. Ex: 1 ... 5 6 7 8 9 ... 20 - would be produced for page 7 with a window size 2 and anchors (1 and 20) enabled. Here is my shared/_paginate.rhtml template:

<% if collection.page_count != collection.first_page -%>
<div class="pagination">
  <ul>
  <% if collection.previous_page? -%>
         <li class="nextpage">
                <%= link_to '&#171; previous', { :page => collection.previous_page } %>
             </li>
      <% else -%>
         <li class="disablepage">&#171; previous</li>
      <% end -%>

    <% last_page = 0 -%>
    <% windowed_pagination_links(collection, :window_size => 2, :link_to_current_page => true, :always_show_anchors => true) do |n| -%>
       <% if collection.page == n -%>
            <li class="currentpage"><%= n %></li>
       <% else -%>
              <li><%= "..." if last_page+1 < n %>
               <%= link_to n, :id => params[:id], :page => n %>
              </li>
           <% end -%>
          <% last_page = n -%>
      <% end -%>

    <% if collection.next_page? -%>
     <li class="nextpage">
        <%=  link_to 'next &#187;', { :page => collection.next_page } %>
     </li>
    <% else -%>
      <li class="disablepage">&#171; next</li>
    <% end -%>
  </ul>
</div>
<% end -%>

Now, add some css magic and we get:

Pretty (excuse my doodles, I need to configure my tablet), fast and efficient. This helper also works with: Eager find_by_sql Pagination and Ferret Pagination in Rails

Ilya GrigorikIlya Grigorik is a web ecosystem engineer, author of High Performance Browser Networking (O'Reilly), and Principal Engineer at Shopify — follow on Twitter.