How to conditionally filter the presentation of category when creating a new category/article?

Question

I am looking for a solution for the following requirements:

1. Display only the category of first level when creating new category.
2. Display only the category of second level when creating article in Joomla.

Answer 1

What you could do is create a couple of custom form fields which restrict the categories in the way you want. Then write a plugin which will trigger on onContentPrepareForm and change the relevant form fields to the custom ones.

Start with the existing category form field code in administrator/components/com_categories/models/fields/categoryedit.php.

You need to change 3 things -

1. Change the class name - eg use JFormFieldCategoryEdit1
2. Change the field type - eg public $type = 'CategoryEdit1';
3. Add an extra where clause to the query in getOptions()

$query->where('a.level = 1'); and level = 2 for the other custom field

Then save the file somewhere - eg in a customisations directory.

Then create a content plugin which will change the type of the category field to the one you want, eg:

 public function onContentPrepareForm(JForm $form, $data)
 {
    if ($form->getName() == "com_content.article" && isset($data->id)) 
    {
        $form->setFieldAttribute("catid", "type", "categoryedit2");
        JForm::addFieldPath(JPATH_ROOT . '/customisations');
    }
    if ($form->getName() == "com_categories.categorycom_content" && !isset($data->id)) 
    {
        $form->setFieldAttribute("parent_id", "type", "categoryedit1");
        JForm::addFieldPath(JPATH_ROOT . '/customisations');
    }
    return true;
}

I tested the code a bit, but not thoroughly, so you may need to change some things. But the general idea works ok.