Unable to get value of default option in select2 jquery

I am using the same form for adding new product and editing product with id. When we add a new product, its categories are selected from the select2 dropdown. But when we edit the already existing product, I want to display its category as default option. I tried with initSelection but in this way only text is displayed and its value is undefined. My form HTML code is:

<form class="form-horizontal form-label-left" novalidate>
    <div class="item form-group">
        <label class="control-label col-md-3 col-sm-3 col-xs-12" >Category 
        </label>
        <div class="col-md-6 col-sm-6 col-xs-12">
           <select class="select2_single form-control col-md-7 col-xs-12" ></select>
        </div>
    </div>
    <div class="item form-group">
        <label class="control-label col-md-3 col-sm-3 col-xs-12" >Name <span class="required">*</span>
        </label>
        <div class="col-md-6 col-sm-6 col-xs-12">
            <input id="name" class="form-control col-md-7 col-xs-12" value="<?php echo $name; ?>"  placeholder="Product Name" required="required" type="text">
        </div>
    </div>
</form>

and my jquery script is

<script>
$(document).ready(function () {         
        categories();
});
function categories() {
   $.ajax({
        'url' : 'category/data', 
        'success' : function(data){
            var j = $.parseJSON(data);
            var select2_ary = [];
            $.each(j, function(k, v) {
                        select2_ary.push({
                              id : v['id'],
                              text : v['category']
                        });    
            });
            var id = "<?php echo $id; ?>";
            if(id != '') {
                  $(".select2_single").select2({
                        placeholder: "Select a Category",
                        initSelection: function(element, callback) {
                            callback({val: '3223', text: '<?php echo $id; ?>' });
                        },
                        data : select2_ary
                   });
            }
            else {
                  $(".select2_single").select2({
                        placeholder: "Select a Category",
                        data : select2_ary
                  });
            }

    }
});


}

</script>

I'm not sure about about the select2 version but its not the latest one. When I submit this form, I am not getting the value of select2 default option. Please help!

you can try this: $(".select2_single").select2({ placeholder: "Select a Category", data : select2_ary }).select2("val",'3223'); });