Do I need a custom field for a db field alias?

Question

When viewing an individual record, I'm using a calculated field from a sql query to display as a read only field in my form. However I'm unable to get it to display (no error, just blank). Curious, do I need a custom form to display this?

/forms/myform.xml

        <field
            name="current_age"
            type="text"
            readonly="true"
            label="DOB_AGE"
            description="DOB_AGE_DESC"
            size="40"
            default="0"
            />

fields/myfield.php

    protected function getOptions()
{
    $db    = JFactory::getDBO();
    $query = $db->getQuery(true);
    $query->select('d.*,(YEAR(CURDATE())-YEAR(d.dancer_birthdate)) - (RIGHT(CURDATE(),5)<RIGHT(d.dancer_birthdate,5)) AS d.current_age,c.title as category');
    $query->from('#__registration_dancers as d');
    $query->leftJoin('#__categories as c on d.catid=c.id');
    // Retrieve only published items
    $query->where('d.published = 1');
    $db->setQuery((string) $query);
    $messages = $db->loadObjectList();
    $options  = array();

    if ($messages)
    {
        foreach ($messages as $message)
        {
            $options[] = JHtml::_('select.option', $message->dancer_id, $message->dancer_name .
                                  ($message->catid ? ' (' . $message->category . ')' : ''), $message->dancer_birthdate,
                                $message->studio_id, $message->dateadded, $message->datemodified, 
                                  $message->current_age, $message->published);
        }
    }

Answer 1

Yes, as far as I know, SQL can only be used with SQL form field and this generates dropdown list. So to just show some value from DB, you have to create custom field.

Old answer

Your need to add the field path to myform.xml parent <fieldset> to load your custom fields. This is only needed if you haven't added your custom fieldpath in PHP.

<fieldset addfieldpath="/administrator/components/<component name>/models/fields">

To add field path with PHP, so you don't need to add it in every form XML, use (for example in your component main file):

JForm::addFieldPath(JPATH_COMPONENT . '/models/fields');

And in myform.xml set type as your new form field.

    <field
        name="current_age"
        type="myfield" <----- custom field type
        readonly="true"
        label="DOB_AGE"
        description="DOB_AGE_DESC"
        size="40"
        default="0"
        />

And your field class should be named JFormFieldmyfield. Just to make sure you have correct class name.

Further reading Joomla! Docs: Creating a custom form field type.

Answer 2

From all of my research and testing, I don't think this is possible. I ended up solving my quandary with a custom field instead of an alias.