Hello all. When developing my own components and using the core Categories system, I have come across a need to have the ability to filter the default Categories dropdown list by a Parent Category. So I figured I'd share this PR in hopes it can be incorporated into the project.

Summary of Changes

Only one file was changed (libraries/cms/html/category.php). Updated the query statement to allow for filter.parent which would filter the categories based off of a specific Parent Category ID number (or an array of parent ID numbers).

With this change a developer can now use some code like the following to filter the default Categories dropdown list...

Filter by a single Parent ID number:
JHtml::_('category.options', 'com_customcomponentname', $config = array('filter.published' => array(1), 'filter.parent_id' => 15))

Filter by multiple Parent ID numbers:
JHtml::_('category.options', 'com_customcomponentname', $config = array('filter.published' => array(1), 'filter.parent_id' => array(15,37)))

Testing Instructions

This can be easily tested using a custom component, specifically when doing a front-end view. Here is an example Categories setup for the custom component:

Books
– Books Subcategory 1
– Books Subcategory 2
Movies
– Movies Subcategory 1
– Movies Subcategory 2
Toys
– Toys Subcategory 1
– Toys Subcategory 2

Now previously if you used JHtml::_('category.options') to do the dropdown list of Categories, all of the Categories for the Custom Component would be listed. So the big list above would be shown. But what if you had a front-end view for Movies? The Books and Toys categories would be shown too. A bit odd and confusing for end users.

With this PR in place, you can now add filter.parent_id to the $config array, like the following code, to show only the Movies categories when on the Movies front-end view...

<select id="filter_category" name="filter_category" class="input-block-level" onchange="this.form.submit()">
  <option value=""><?php echo JText::_('- Show All Movies -'); ?></option>
  <?php echo JHtml::_('select.options', JHtml::_('category.options', 'com_customcomponentname', $config = array('filter.published' => array(1), 'filter.parent_id' => 15)), 'value', 'text', $this->state->get('filter.category')); ?>
</select>

Just change parent_id 15 to whatever ID number it is in your setup and your Categories dropdown will now only show...

– Movies Subcategory 1
– Movies Subcategory 2

No more un-relevant categories listed on specific views and no more confusion for the users :)

Documentation Changes Required

I would imagine that some documentation somewhere would need updating. I'd be happy to contribute as needed if folks would like.