Selectable optgroup using select2

This is possible if you back the Select2 with a hidden input element — instead of a select element.

To make a group option selectable, you must give it an “id”, but it appears it can be an empty string. You can then use the “select2-selecting” event to prevent the group option from getting selected, and instead have its child options get selected.

Additionally, a query function can be provided to prevent a group option from appearing in the list after all its child options have been selected.

If you have options defined like this:

var FRUIT_GROUPS = [
    {
        id: '',
        text: 'Citrus',
        children: [
            { id: 'c1', text: 'Grapefruit' },
            { id: 'c2', text: 'Orange' },
            { id: 'c3', text: 'Lemon' },
            { id: 'c4', text: 'Lime' }
        ]
    },
    {
        id: '',
        text: 'Other',
        children: [
            { id: 'o1', text: 'Apple' },
            { id: 'o2', text: 'Mango' },
            { id: 'o3', text: 'Banana' }
        ]
    }
];

You can instrument your Select2 like this:

$('#fruitSelect').select2({
    multiple: true,
    placeholder: "Select fruits...",
    data: FRUIT_GROUPS,
    query: function(options) {
        var selectedIds = options.element.select2('val');
        var selectableGroups = $.map(this.data, function(group) {
            var areChildrenAllSelected = true;
            $.each(group.children, function(i, child) {
                if (selectedIds.indexOf(child.id) < 0) {
                    areChildrenAllSelected = false;
                    return false; // Short-circuit $.each()
                }
            });
            return !areChildrenAllSelected ? group : null;
        });
        options.callback({ results: selectableGroups });
    }
}).on('select2-selecting', function(e) {
    var $select = $(this);
    if (e.val == '') { // Assume only groups have an empty id
        e.preventDefault();
        $select.select2('data', $select.select2('data').concat(e.choice.children));
        $select.select2('close');
    }
});

jsfiddle

Here is a jsfiddle without the query function, where the group options still appear when all of their child options are selected.