Using form XML for custom filter and removing group definition

Question

I'm creating a small component with a search module. I will dumb this example down so that the question will be clear.

The component manages books. The book form XML looks like this and its structure CANNOT be changed:

<?xml version="1.0" encoding="utf-8"?>
<form>
    <fieldset addfieldpath="/administrator/components/com_books/models/fields">
        <field name="title" type="text" default="" />
        <field name="type" type="text" default="" />
        ...
        <fields name="specs">
            ...
            <field name="category" type="category" label="Category">
                <option value="">JSELECT</option>
            </field>
            ...
        </fields>
    </fieldset>
</form>      

The category field is a custom predefined list:

use Joomla\CMS\Factory;
use Joomla\CMS\Form\FormHelper;

FormHelper::loadFieldClass('predefinedlist');

class JFormFieldCategory extends \JFormFieldPredefinedList
{
    public $type = 'Category';

    protected $predefinedOptions = array(
        '1' => 'Cat1',
        '2' => 'Cat2',
        '3' => 'Cat3',
        ...

    );
}

The component works fine (MVC) with selecting the category list and saving.

Now for the search module... I want to use a select filter in the module which is based on the category field.

What's the leanest way to do this? I don't want to repeat the category options. At the moment, I have this in the default.php of the module:

$value = Factory::getApplication()->input->getInt('category');
$bookform = Form::getInstance('com_books.book', 'book');
$bookform->setFieldAttribute('category', 'class', 'customclass', 'specs');
$bookform->setValue('category', 'specs', $value);
echo $bookform->getInput('category', 'specs', $value);

If I do this I get the following element:

<select id="specs_category" name="specs[category]" class="customclass">
    <option value="">JSELECT</option>
    <option value="1">Cat1</option>
    <option value="2">Cat2</option>
    <option value="3">Cat3</option>
    ...
</select>

But I don't want the group name inside. I know I can change the keying like this:

$bookform = Form::getInstance('com_books.book', 'book', array('control' => 'custom'));

<select id="specs_category" name="custom[category]" class="customclass">

But this is not working:

$bookform = Form::getInstance('com_books.book', 'book', array('control' => false));

What I need is this:

<select id="specs_category" name="category" class="customclass">

And please consider I cannot change the book XML structure as it is predefined for my work. Also this example is a bit dumbed down, so I cannot use custom fields.

I tried adding this to the custom category field

public function setup(\SimpleXMLElement $element, $value, $group = null)
{
    $result = parent::setup($element, $value, null); // the null removes the keying
    return $result;
}

That works, but the keying is needed when editing a book or when creating a menuitem, that uses this field as an option and the group params is needed.

I know I could get the HTML and fiddle around with a regex to remove the keying, but I'm sure that there is a better way.

Answer 1

So this is what I ended up doing:

$value = Factory::getApplication()->input->getInt('category');
$bookform = Form::getInstance('com_books.book', 'book');

$field = $bookform ->getField('category', 'specs');
$field->__set('group', null);
$field->__set('name', 'category');
$field->__set('class', 'customclass');
$field->setValue($value);

echo $field->renderField(array('customoption1' => true));

Would be nice to find a more lean solution if possible.