Foreign Key select from existing components

<?php
/**
* @author        DDCS, Limited
* @copyright    2014; All Rights Reserved
* @license        Commercial International License; See LICENSE.txt
*/

defined("_JEXEC") or die("Restricted access");

defined('JPATH_BASE') or die();
JFormHelper::loadFieldClass('list');

/**
* Describe this field
*/
class JFormFieldTherapistList extends JFormFieldList
{
    /**
    * The form field type.
    *
    * @var    string
    * @since  11.1
    */
    protected $type = 'therapistlist';

    /**
    * Method to get the field options.
    *
    * @return  array  The field option objects.
    *
    * @since   11.1
    */
    protected function getOptions()
    {

        // The HTML option elements - do not edit this one
        $options = array();

// Get a list of all tables currently in the database. 
// getTableList returns an array, which is helpful
        $tables      = JFactory::getDbo()->getTableList();
        
// Set the name of the table you are looking for. Because you have no idea what the prefix will be,
// just use the tablename part of the name string. The Joomla #__ variable does not seem to work here
// for some reason I do not care to investigate.
        $gettable    = 'therapists';
        
// find out if your table actually exists in the database. I do this by using the substring count function
// and imploding the array of table names into a string using implode()
        $table_check = substr_count ( implode( $tables ), $gettable );
        
// Now that you know if the table is installed, you can get the option data from the table. 
        if($table_check != 0 )
        {

            $db    = JFactory::getDbo();
            $query = $db->getQuery(true);
            $query->select('id, name');
            $query->from('#__therapists');
            $query->where('`state` = 1');
            $db->setQuery($query);
            $alltherapists = $db->loadObjectList();

// Create a new option object based on the < option /> element.

// Put in a default message value for the top of the options list.
            $options[] = JHtml::_('select.option', '', 'Select a Therapist');

// Now loop through the Query results and add an option for each value
            foreach($alltherapists as $therapist){
                // Create options for the < option /> element.
                $options[] = JHtml::_('select.option', $therapist->id, $therapist->name);

            }
        }
// If the table does not exist you can simply put in a "Sorry, Charlie" type of message.
        else
        {
            // Create a new option object based on the < option /> element.
            $options[] = JHtml::_('select.option', '', 'Therapist List Not Available');

        }

// Merge any additional options in the XML definition.
        $options = array_merge(parent::getOptions(), $options);

// Return the options you have found.
        return $options;
    }


    public function getInput()
    {

        // The HTML option elements - do not edit this one
        $options = array();

        $options = $this->getOptions();

        return JHTML::_('select.genericlist', $options, $name    = $this->name, $attribs = null, $key      = 'value', $text     = 'text',  $selected = $this->value);
    }

}

<field name="therapist" type="therapistList" size="11" class="inputbox"
        
            label="COM_PATIENTS_FORM_LBL_DOCUMENT_THERAPIST"
            description="COM_PATIENTS_FORM_DESC_DOCUMENT_THERAPIST" 
        />