mysite_code_MemberAutocompleteField.php

<?php

class MemberAutocompleteField extends TextField {

	private static $allowed_actions = array('suggest');

	public $maxSuggestionsNum = 50;

	public function Field($properties = array()) {
		Requirements::javascript("mysite/javascript/MemberAutocompleteField.js");
		return parent::Field($properties);
	}

	public function getAttributes() {
		$attrs = parent::getAttributes();
		$custom = array(
			'class' => $attrs['class'] . ' member-autocomplete text',
			'data-source' => parse_url($this->Link(), PHP_URL_PATH).'/suggest'
		);

		return array_merge($attrs, $custom);
	}

	public function suggest($request) {
		if (!Permission::check('CMS_ACCESS_LeftAndMain')) return Convert::raw2json(array());

		$searchString = trim($request->requestVar('term'));

		//NOTE: using the LOWER function will mean that indexes will not work on this column.
		$SQL_filter = 'LOWER(' . sprintf("\"Member\".\"Email\") LIKE LOWER('%%%s%%')",
				Convert::raw2sql($searchString)
			);

		$tagObjs = DataObject::get('Member', $SQL_filter, $this->tagSort, "", $this->maxSuggestionsNum);
		$tagArr = ($tagObjs) ? array_values($tagObjs->map('ID', 'Email')->toArray()) : array();

		return Convert::raw2json($tagArr);
	}

	public function dataValue() {
		return $this->value ? Member::get()->filter('Email', $this->value)->first()->ID : null;
	}

	function setValue($value, $obj = null) {
		if ($value && is_numeric($value)) $value = Member::get()->byID($value)->Email;
		parent::setValue($value, $obj);
	}
}

mysite_javascript_MemberAutocompleteField.js

(function($){

	$('input.member-autocomplete').entwine({
		onadd: function(){
			var source = this.attr('data-source');

			this.autocomplete({
				source: source,
				minLength: 3
			});
		}
	})


}(jQuery))