The server on which my weblog used to run is getting rather old and crumbly, and brings with it a constant low-level dread that some day, real soon now, it’s going to give up the ghost. So for the last while, we’ve gradually been moving everything off it. When it came time to move this site, I decided that it was also a good opportunity to migrate away from Movable Type, mostly for the same philosophical reasons that Mark Pilgrim has already set out.

I considered moving to Typo, following Piers‘ lead, mostly just so I could play with Ruby, but in the end I decided on WordPress. I’ve made the mistake too many times now of choosing software based on the language in which it’s written. Yes, it would be nicer to be able to hack on my weblog in Ruby or Perl than in PHP, but I know enough PHP to get by, and I doubt I’ll be doing that much hacking anyway.

Setting the weblog up was fairly trivial, as most good PHP installations tend to be. Migrating all my old content wasn’t quite so simple. WordPress 2 seems to have made the import process much simpler than before; most of the information on the process I’ve found relates to older versions and isn’t really applicable any more. Unfortunately, although the simple case of importing my MT archive was fairly painless, I really didn’t want to break all my old links.

There are a few sites that discuss how to maintain your Movable Type post IDs, but they all seem to relate to the old WordPress process. So I had to get my hands dirty in PHP much quicker than expected.

Firstly, I had to edit MT/App/CMS.pm in my MT setup, adding a line to include the entry id in the export output:

AUTHOR: <$MTEntryAuthor$>
TITLE: <$MTEntryTitle$>
ID: <$MTEntryID$>
STATUS: <$MTEntryStatus$>

Then I was able to export all my posts.

I had to post-process the output file, however, as I’ve been creating my posts using the MT Kwiki plugin. This meant that none of my links imported correctly. I spent much too long wrestling with vim’s non-greedy regular expressions before giving up and processing the data in Perl instead:

perl -pe 's/[(http:.*?) (.*?)]/$2/g' mt-dump.txt |
perl -pe 's/[(.*?) (http:.*?)]/$1/g' > deWikied.txt

Then I had to persuade WordPress to maintain the MT ids. In the old WordPress import script it just inserted the posts by hand, and it was a simple matter of ‘fixing’ the SQL it used to do this. But now the importer calls the same code that is used when you create a post through the normal interface.

So I needed to add a check for the ID into the import/mt.php script:

case 'AUTHOR' :
    $post_author = $value;
    break;
case 'ID' :
    $post_ID = $value;
    break;

And then fix the call that inserts the data:

$postdata = compact('post_ID','post_author', 'post_date', 'post_date_gmt', ... );

Then I needed to adjust the wp_insert_post() call to cope with an incoming post_ID:

  if ( !isset($post_ID) )
      $post_ID = 0;
  if ( !isset($post_password) )
      $post_password = '';

and adjust its SQL accordingly

"INSERT IGNORE INTO $wpdb->posts (id, post_author, post_date, ... ) VALUES
  ($post_ID, '$post_author', '$post_date', ...)");

(The arguments are passed as an extract() of a get_object_vars(), so there’s no need to change any of the other handling).

I believe that this is a safe enough approach that won’t interfere with creating new posts or editing old ones, but you can always revert this file back after importing if there are any problems.

With this in place, I was able to import all my old posts. (There were a lot of them, so I actually had to split the file and import 4 segments in turn). The other thing that the docs don’t make clear is that you need to have an upload directory which is writable by your webserver, but that was easy enough to work out from the error message.

They all came in with the same IDs as they used to have, so then it was just a matter of setting up some Apache redirects on the old server:

Redirect permanent /nothing/index.rdf
http://nothing.tmtm.com/feed/
RedirectMatch permanent /nothing/archives/([0-9]{6}).html
http://nothing.tmtm.com/archives/$1
RedirectMatch permanent /nothing/archives/([0-9]{4})_([0-9]{2}).html
http://nothing.tmtm.com/archives/date/$1/$2
RedirectMatch permanent /nothing/archives/([0-9]{4})_([0-9]{2})_([0-9]{2}).html
http://nothing.tmtm.com/archives/date/$1/$2/$3
RedirectMatch permanent /nothing/archives/([0-9]{4})_([0-9]{2})_([0-9]{2}).html
http://nothing.tmtm.com/archives/date/$1/$2/$3

(I’ve already changed my permalink structure in WordPress to have this style of URL)

There will be many more things to change later to replicate the changes I’d made to my MT set-up, but this at least gets me up and running on WordPress.