Next and Previous - Within a Category

MT automatically gives you the next and previous items for your individual archives with <MTEntryNext> and <MTEntryPrevious> items. The problem is that's within the global list of all entries within your blog. What if you wanted to link to the next or previous item within a given category?

As an example, check out any of the entries on Forwarded Funnies. There, in the upper right, you'll see not only a previous and next for "All Entries", but a separate previous and next for entries of the same rating as the current. In other words a previous and next for all the "G-Rated" entries if the current item is itself G-rated.

Needless to say, ratings within Forwarded Funnies are simply separate MT categories.

My approach uses a combination of PHP code generated from a category archive template, and a server-side include, or "SSI", in the individual entry archive template.

The category archive template I added is this:

<?
    $szID = $_GET['id'];
    $szOp = $_GET['op'];
    $szText = $_GET['text'];

    <MTEntries sort_order="ascend">
     $rgArchive[<$MTEntryID$>] = "<$MTEntryPermalink$>";
     $rgTitle[<$MTEntryID$>] = <<<H<$MTEntryID$>
<$MTEntryTitle$>
H<$MTEntryID$>;
    </MTEntries>

    if (array_key_exists ($szID, $rgArchive))
      {
      while (key ($rgArchive) != $szID)
        {
        $szPrev = key ($rgArchive);
        next ($rgArchive);
        }
      $szNext = next ($rgArchive);
      $szNext = key ($rgArchive);
      
      if (("next" == $szOp) && ("" != $szNext))
        echo "$szText <a href=\"" . $rgArchive[$szNext] 
            . "\">" . $rgTitle[$szNext] . "</a>";
      elseif (("prev" == $szOp) && ("" != $szPrev))
        echo "$szText <a href=\"" . $rgArchive[$szPrev]
            . "\">" . $rgTitle[$szPrev] . "</a>";
      }
?>

The MT Entries section generates PHP code such as this:

  $rgArchive[500] = "http://www.forwardedfunnies.com/do_not_be_late_000500.html";
  $rgTitle[500] = <<<H500
Do Not Be Late
H500;
  $rgArchive[501] = "http://www.forwardedfunnies.com/jesus_mary_joseph_and__000501.html";
  $rgTitle[501] = <<<H501
Jesus, Mary, Joseph and ...
H501;

In other words, it defines two arrays that map the item's ID to its Title and Permalink. When used, this PHP code takes an entry ID as a parameter, and then outputs a link to either the next or previous item in the list.

We'll see exactly how that's used in just a second. First, I have to make sure that the files generated when this template is built are named appropriately.

In the weblog's Settings, in the Publishing tab, down in Archive Mapping, I created a new Archive Mapping for our new template. It is, as we said, a Category archive, using the new template we added above.

Most importantly, I defined the Archive File Path to be: "catnav_%c.php"

This will then cause a category navigator, as I call it, to be built for each category defined in the system. For example:

catnav_grated.php
catnav_pgrated.php
...

Now, to hook it up with the individual entries.

In the individual entry template, I place this code:

<MTEntryCategories>
  <br />
  <a href="<$MTCategoryArchiveLink$>"><b><$MTCategoryLabel$></b></a><b> Entries:</b><br />
  <!--#include VIRTUAL="catnav_<$MTCategoryLabel dirify="1" $>.php?id=<$MTEntryID$>&op=next&text=Next:" --><br />
  <!--#include VIRTUAL="catnav_<$MTCategoryLabel dirify="1" $>.php?id=<$MTEntryID$>&op=prev&text=Previous:" --><br />
</MTEntryCategories>

There are several interesting parts to this fragment, so I'll go over them one by one.

First, note the use of "<MTEntryCategories>". While within Forwarded Funnies, each item is assigned only one category, that's not a required limitation. So the code that we'll generate here will be repeated once for each category that this entry is assigned.

I start with a link to "<$MTCategoryArchiveLink$>" - that's a link to the standard category archive page that simply lists all entries that have been assigned the category. Nothing really new here.

I invoke the appropiate "catnav_??.php" within an SSI. This requires a little more detailed explanation:

• "catnav_<$MTCategoryLabel dirify="1" $>.php" gives me the name of the category navigator for the current category, dirified so that improper characters are removed. A G-Rated item will result in "catnav_grated.php" being the file processed by the SSI command.

• The ".php" file is passed parameters as part of its execution by the SSI.

• "id=<$MTEntryID$>" passes the ID of the current entry to the .php script.

• "op=next" or "op=prev" tells the script whether to get the next or the previous entry relative to the current entry.

• "text=Next:" or "text=Previous:" is additional text that the script should include, but only if it does, in fact, output an entry. There is no next for the last item, and no previous for the first, in which case this text is not output.

Note that I include the SSI twice - once with parameters indicating that the next item be fetched, and once again for the previous.

When all is said and done, each entry has a next and previous link within their category, something like this:

G-Rated Entries:
Next: Horse For Sale
Previous: Colorado Barbie

Or, in HTML:

<a href="http://www.forwardedfunnies.com/grated.html"><b>G-Rated</b></a><b> Entries:</b><br />
Next: <a href="http://www.forwardedfunnies.com/horse_for_sale_009802.html">Horse For Sale</a><br />
Previous: <a href="http://www.forwardedfunnies.com/colorado_barbie_009819.html">Colorado Barbie</a><br />

Posted February 18, 2006