Quickie fieldtype for a dropdown where each value can only be selected in a single entry.

ft.distinct_dropdown.php

<?php if (! defined('BASEPATH')) exit('No direct script access allowed');

/*
	Distinct Dropdown fieldtype
	- Requires the P&T fieldpack to be installed
	- Won't work inside a Matrix row
	- Won't work in SafeCracker, as we can't access the current entry_id
	(at least I don't think?)
	- Idea could be ported to multiselect, checkbox group, and radio fields
*/

if (! class_exists('Pt_dropdown_ft'))
{
	require PATH_THIRD.'pt_dropdown/ft.pt_dropdown.php';
}

class Distinct_dropdown_ft extends Pt_dropdown_ft {

	var $info = array(
		'name'     => 'Distinct Dropdown',
		'version'  => '1.0'
	);

	function _display_field($data, $field_name)
	{
		$this->prep_field_data($data);
		
		// Has this value already been used?
		$sql = "SELECT $this->field_name
		FROM exp_channel_data 
		WHERE $this->field_name != '' AND $field_name != '--'";

		// Are we editing an existing entry?
		if($entry_id = $this->EE->input->get('entry_id'))
		{
			$sql .= " AND entry_id != '".$this->EE->db->escape_str($entry_id)."'";
		}
				
		$existing = $this->EE->db->query($sql);
		if($existing->num_rows() > 0)
		{
			$js = "$('";
			foreach($existing->result_array() as $row)
			{
				$val = $row[$field_name];
				$js .= 'select[name="'.$this->field_name.'"] option[value="'.$val.'"], ';
				// Instead of setting "disabled" via JS, we could do this:
				// unset($this->settings['options'][$data]);
				// But it's nicer to see the option disabled
			}
			$js = rtrim($js, ', ');
			$js .= "').attr('disabled', 'disabled');";
			$this->EE->javascript->output($js);
		}
		
		// Add a nullifier so that if we've selected all of our options
		// We have something to deselect to
		$options = array_merge(array('--' => '--'),  $this->settings['options']);
		
		return form_dropdown($field_name, $options, $data);
	}
}