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From the author of Nesting Components with Joomla Categories

Nesting Components with Joomla Categories

Joomla 2.5 includes a very powerful built-in category system that supports nesting categories in a hierarchy and setting ACL permissions as the category level. If you're developing a component extension for Joomla, consider using this category system to allow your component items to be grouped by category.

The great thing is that you can do this with just a few lines of code. For example, the core Weblinks component displays the Categories link in the pull-down menu (see Figure 7).

Figure 7 Category Manager menu example in Joomla back end.

The code for this capability actually comes from the installation XML file, in this case administrator/components/com_weblinks/weblinks.xml. Here it is:

<menu link="option=com_categories&amp;extension=com_weblinks"
   view="categories" img="class:weblinks-cat"

Notice the link to com_categories. That's the built-in core component that handles all of the work for categories. All you need to do is to create a link with your extension name in the link—in this case, extension=com_weblinks.

We also want a way to navigate to the category manager screen from the component manager screen. Figure 8 shows how it looks in Weblinks.

Figure 8 Categories tab in Joomla back end.

Here's all of the code for this (from the file administrator/components/com_weblinks/helpers/weblinks.php):

   $vName == 'categories'
if ($vName=='categories') {
      JText::sprintf('COM_CATEGORIES_CATEGORIES_TITLE', JText::_('com_weblinks')),

This is called from the file display() method of administrator/components/com_weblinks/controller.php, as follows:

// Load the submenu.
WeblinksHelper::addSubmenu(JRequest::getCmd('view', 'weblinks'));

That's it! With these few lines of code, we've added all the power of nested categories to the component.

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