When a site gets to a certain scale, storing categories on a per-blog basis starts to get a little messy. Assuming someone would want all WPMU blogs to share a single set of categories, how might one go about that? My gut it so store a set of "cannonical" categories in the root blog. Is there a more elegant or standardized solution?

4 Answers 4


A more elegant solution would be to create a MU (must use) plug-in and drop it on the network. This plug-in would check (per-site) if the categories exist and, if not, add them as appropriate.

Here's some untested example code:

$default_categories = array(

foreach($default_categories as $cat) {
    if( get_cat_ID( $cat ) != 0 ) continue;

    wp_insert_term( $cat, 'category' );

This should loop through your list of categories and attempt to fetch the ID of each. If the category doesn't exist, get_cat_ID() will return "0" so you know to insert the category. This code won't set the slug, description, parent, etc ... I leave that as an exercise for you to complete.


Note: I'm assuming you want the categories to only be stored in one place...

If it was me and I had to do that I'd make an mu-plugin that hooks into the taxonomy insert/update/edit methods.

I'd create a new set of tables copied from the existing taxonomy tables for the parent site (just in case), with an additional field to indicate which site a post_id belongs to. The plugin would then be used to remap the name of the taxonomies table to our new site-wide one.

You'd then need a filter on all db queries to the taxonomical relationships to make them match the current site's id.

You'd also need to remove people's privileges to add/remove categories unless you're a glutton for punishment.

In short (and IMO) it would be a lot messier and a lot more restrictive to your users to have a single set of categories shared between sites. You may as well have one site with multiple authors.

  • I think trying to tackle this filtering at the query-level is a bad idea. For me it's much easier to just filter or hook existing functionality like the save_post action.
    – editor
    Commented Jan 11, 2011 at 19:27

Not sure if this solves your question, but perhaps my plugin bbAggregate addresses your requirements? It allows you to create 'streams' of content from multiple blogs in a multisite WordPress install and aggregate them onto a page. It's my replacement for Global Terms ;)

Have a look at the plugin: http://wordpress.org/extend/plugins/bbaggregate/

Read this article about it: www.roytanck.com/2010/09/30/bbaggregate-lets-you-mix-and-match-wordpress-content/

Feel free to contact me if you have any questions and/or remarks related to this plugin,


Hi @editor:

Your question caused me to ponder many issues so I felt that the hackers list might be a good place to ask. Here's the start of the thread; you might find it enlightening:

  • Thanks. We've actually worked this one out, so I know it's possible even if a bit obtuse. (Interestingly, I think someone told me that back in the day WordPress stored its categories in a single table rather than per-blog, so I suspect this problem arose shortly after that schema was dumped.)
    – editor
    Commented Jan 11, 2011 at 19:25

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