$(document).ready(function() {
    function customFilter(heading, filter, text) {
        var filtergroup = $('#changelist-filter h3:contains("'+ heading +'")').next('ul');
        qs = window.location.search;

        // if we're already filtering by a sibling (that isn't "All")
        if (filtergroup.children('li:first').siblings().is('.selected')) {
            // remove sibling querystring from custom filter querystring
            var filterBase = filter.substr(0, filter.indexOf('_'));
            var siblingQs = qs.substr(qs.indexOf(filterBase), qs.indexOf('&', qs.indexOf(filterBase)));
            if (!siblingQs) {
                siblingQs = qs.slice(qs.indexOf(filterBase), qs.length);
            }
            qs = qs.replace(siblingQs, '');
            qs = qs + '&' + filter;
        } else {
            // no current filters, make new
            if (!qs || qs == '?') {
                qs = '?' + filter;
            // already filtering by custom filter, ignore
            } else if (qs.match(filter)) {
                qs = qs;
            // current filters, append
            } else {
                qs = qs + '&' + filter;
            }
        }

        filtergroup.append('<li><a href="'+ qs +'">'+ text +'</a></li>');

        var currentfilter = filtergroup.children('li:last');
        if (window.location.search.match(filter)) {
            currentfilter.addClass('selected');
            currentfilter.siblings().each(function(){
                $(this).removeClass('selected');
                $(this).children('a').attr('href', function() {
                    return this.href.replace(filter, '');
                });
            });
        }
    }

    // Usage examples. Takes three arguments.
    // 1. the exact text above your list_display section
    // 2. the custom field lookup (careful with weird characters here)
    // 3. the text you want the new filter to show up as
    customFilter('By affiliation', 'school__in=A,B', 'School A or B');
    customFilter('By affiliation', 'school__isnull=True', 'No school');
});