Elin Waring has started work on a tagging system for Joomla!:
Re: [jcms] Re: Multiple Categories
Mark and I talked a lot about this topic since the last post. I'm going to give a summary of what I see as the goals for this and why but if you want to skip ahead there is a code link at the bottom.
So I would describe what as wanted as this: 1. a way of grouping items that allows multiple assignments to the same item and 2. for these things that for simplicity sake I am going to call tags should optionally be able to be nested. 3. It should be easy to make a menu link (or a link in general) that gets all of the items with a certain tag and 4. optimally the tags should be able to apply to any content type for which the developer buys into the system.
Tags are different from what we have in Joomla categories in that: tags are NOT containers for other items meaning the hierarchy of tags end with tags and does not have final leaves that are items like articles. Items do not inherit anything from a tag whereas with categories items (and other categories) inherit acl and publication status. That is a tag being unpublished or limited to managers does not limit the items tagged with it to being unpublished or to visibility.
Some of the confusion about terminology here I think comes from people looking at WP which has what I call tagegories. That is it has a combined system with a single table that holds categories, which can be nested, and tags, which can't be nested. The tags concept came after the categories concept basically because of problems with scaling when there are large numbers of categories, which is why WP encourages users to limit use of categories and focus on tags instead. Basically WP uses a system for categories that is similar to what we had in 1.5 or what we have for mappings in smart search. That is it does not use nested sets which is why it doesn't scale very well.
So what I did was to write some code for a new core component to create and manage tags and allow applying tags to the four core content types (though there is no reason that categories and user profiles couldn't be added to the list, but this is just potentially phase one if we decide to go for this). It definitely is not finished in that some people who are great at JQuery would want to make the ui better especially focused on an easy way to create new tags on the fly when editing/creating and item and I didn't modify layouts to show the tags an item is assigned to (the new layouts system will be good for this), but it's a start. So if you are interested please take a look and test and give some feed back.
http://joomlacode.org/gf/project/joomla ... m_id=29855
If you have experience in testing patches, please test this!